REVIEW PAPER

Clogging in horizontal subsurface flow constructedwetlands: influencing factors, research methodsand remediation techniques

Mateus Pimentel de Matos . Marcos von Sperling . Antonio Teixeira de Matos

Published online: 25 January 2018

� Springer Science+Business Media B.V., part of Springer Nature 2018

Abstract The treatment of wastewater in con-

structed wetlands (CW) has been increasingly applied

throughout the world, as it is an efficient technique for

the removal of pollutants and presents low construc-

tion and operational costs. However, a major opera-

tional problem of these systems is clogging of the

porous medium. Clogging of CW has therefore

attracted the attention in several studies, but there

are several gaps in the understanding of this phe-

nomenon, especially with regards to its genesis. In

order to evaluate the contribution of the influencing

factors and to facilitate remediation, it is important to

have methods that favor characterization of the real

conditions of CW. In this review, the objective was to

gather information on the main factors interfering in

the clogging process of horizontal subsurface flow

constructed wetlands, the available and the new

methods for characterizing the degree of obstruction

of the porous medium and the techniques/strategies for

unclogging these systems.

Keywords Bulking factor � Hydraulic conductivity �Wear � Unclogging � Swelling � Plants

1 Constructed wetlands

Constructed wetlands (CWs) are treatment units,

which simulate natural flooded areas, and have

mechanisms to remove pollutants involving the

medium-plant-microorganisms system. Among the

different configurations and operating conditions of

reactors, the units can be divided into three groups:

surface flow constructed wetlands (SF-CW), vertical

subsurface (VSSF-CW) and horizontal subsurface

(HSSF-CW) flow constructed wetlands (Fonder and

Headley 2013).

Several studies have been carried out in CWs,

highlighting their great efficiency in the removal of

different pollutants, treating different types of wastew-

ater, their low construction and operation costs and the

generation of a green mass that can be used after

cutting (Vymazal 2005; Pedescoll et al. 2009). How-

ever, like any treatment system, they also have

limitations, such as dependence to environmental

factors, the large area demanded, and be subject to

clogging of the porous space (Dotro et al. 2017). A

M. P. de Matos (&)

Department of Engineering, Nucleus of Environmental

and Sanitary Engineering, Federal University of Lavras,

Lavras, Minas Gerais, Brazil

e-mail: mateus.matos@deg.ufla.br

M. von Sperling � A. T. de Matos

Department of Sanitary and Environmental Engineering,

Federal University of Minas Gerais, Belo Horizonte,

Minas Gerais, Brazil

e-mail: marcos@desa.ufmg.br

A. T. de Matos

e-mail: atmatos@desa.ufmg.br

123

Rev Environ Sci Biotechnol (2018) 17:87–107

https://doi.org/10.1007/s11157-018-9458-1

sequence of clogging potential in conventional con-

figurations of the systems would be: SF-CW\VSSF-

CW\HSSF-CW, due to differences in the loading

regime (continuous or intermittent), presence or

absence of substrate, and position in which the

wastewater is applied (over the surface area or on

the cross-section area) (Kadlec andWallace 2009; Wu

et al. 2015). As a result, this review will address

mainly the clogging process in HSSF-CW, which

usually have shorter useful lives. However, some

discussions about vertical units will also be presented,

aiming at assisting in the discussion of the phe-

nomenon of obstruction of pore spaces and reduction

of clean porosity.

Other review articles have dealt with the occurrence

of the main contributing factors, modeling and reme-

diation of clogging (Knowles et al. 2011; Nivala et al.

2012), emphasizing the importance of this topic. The

present work aims to complement the discussion,

presenting new methods of characterizing the pore

space of CWs, their advantages and disadvantages,

and their possible influence in the evaluation of the

contribution of each factor associated with the clog-

ging phenomenon. It is also the objective to present

new findings regarding the key factors in the clogging

process and bed remediation techniques.

2 Factors associated with clogging

2.1 Introduction

In constructed wetlands (CW), the removal of pollu-

tants occurs by physical, chemical and biological

means. Low flow velocity through the pores favors

sedimentation and filtration of suspended solids and

their adhesion to the porous substrate. The dissolved

solids from the influent are converted to suspended

solids due to the growth and reproduction of microor-

ganisms organized in biofilms (Kadlec and Wallace

2009).

In these systems, the microbial metabolism for

degradation of organic material, with mineralization

and assimilation of nutrients, leads to the formation of

by-products and exudates. In parallel, precipitation of

iron and heavy metals may occur in complexes with

sulfides and carbonates, and phosphorus by precipita-

tion with iron or by co-precipitation with aluminum

(Kadlec andWallace 2009). These phenomena depend

on, among other variables, the redox potential and

characteristics of the filtering material (Tanner and

Sukias 1995; Platzer and Mauch 1997; Rowe et al.

2000; Kadlec and Wallace 2009).

Figure 1 shows the progressive stages of solids

accumulation and clogging in horizontal subsurface

flow constructed wetlands (HSSF-CWs). (1) Over

time, several mechanisms interact in the accumulation

of solids within the porous spaces present in the

substrate, leading to an internal blockage with a

decrease in drainable porosity. With the maturation of

the bed, this makes the medium more restrictive to the

passage of substances, initially increasing the effi-

ciency of the systems (Karathanasis et al. 2003;

Suliman et al. 2006; Xu et al. 2013). (2) However, the

advancement of pore obstruction results in lowering

the hydraulic conductivity (k) in the medium, causing

wastewater to move through less tortuous or less

impeded paths (preferential pathways) (Caselles-Oso-

rio et al. 2007; Nivala and Rousseau 2009; Babatunde

2010). (3) In HSSF-CW, dead zones and preferential

paths are then formed, and in a more advanced stage

the flow that was previously in the subsurface becomes

superficial, especially in the initial section of the bed.

This leads to a reduction of the hydraulic retention

time (HRT) of the wastewater, what may result in

lower physical, chemical and biochemical removal of

the pollutants, and may lead to a decrease in their

efficiency (Rousseau et al. 2005; Nivala and Rousseau

2009; Babatunde 2010). (4) Liquid on top of the bed

may be associated with anaerobic conditions, impair-

ing effectiveness of the treatment and bringing the

potential for generation of bad odors and the prolif-

eration of flies and other insects (Fu et al. 2013). It is

observed in Fig. 1 that clogging is gradual, with the

most critical conditions near the inlet end as indicated

in a cross-section of the HSSF-CW, where it receives a

higher organic load. The movement of the liquid

inside the porous spaces is simplified in the figure (-

tortuosity, mechanical dispersion, among other fac-

tors), to focus only on the effect of the accumulation of

solids in the porous space.

The clogging phenomenon, widely discussed, has

not yet been fully established with regards to its cause

and characterization. There is no consensus if it is a

phenomenon that can be subdivided into successive

phases of progression (De la Varga et al. 2013), if it is

associated with the appearance of surface flow or

collapse of the unit, with an identifiable loss of

88 Rev Environ Sci Biotechnol (2018) 17:87–107

123

efficiency. Despite this, it must be recognized as a

serious operational problem, and therefore should be

studied so that techniques can be made available that

seek to attenuate and/or remediate the problem,

providing a longer life of the system.

Previous studies estimated the useful life of CW in

50–100 years, which was later reduced to 15 years

(Nivala et al. 2012). More recently, reports indicate

that the system operates without hydraulic collapse for

periods less than 10 years (Knowles et al. 2011), such

as 8 years as reported by Griffin et al. (2008), although

it is common to observe an advanced degree of

clogging after 2 (Paoli and von Sperling 2013a) or

4 years (Caselles-Osorio et al. 2007; Nivala and

Rousseau 2009).

According to Grismer et al. (2001), after 4 months

of operation clogging begins to influence the hydro-

dynamic conditions of the CW, and it presents a

significant effect after about 2 years according to

Tanner et al. (1998a), after which time the

hydrodynamic conditions would stabilize. On the

other hand, Kadlec and Wallace (2009) considered

that flow is continuously influenced by the degree of

obstruction in the CW, which would have a continuous

decrease in HRT.

2.2 Factors causing clogging

Clogging is still a large ‘‘black box’’, where the main

factors that cause it are not well known. There are also

few details and many possibilities for evaluating the

pore clogging process that are dependent on the

characteristics of the wastewater and the medium used

to treat it. The factors most cited in literature that cause

clogging are suspended solids, biofilm, precipitates,

gases, wear of the support material (substrate), roots,

rhizomes and plant debris.

Based on what was described in several publica-

tions, a schematic illustration was produced to

Fig. 1 Progression stages of solids accumulation and clogging in the HSSF-CWs

Rev Environ Sci Biotechnol (2018) 17:87–107 89

123

summarize the factors involved in the clogging

process and their contributions, as shown in Fig. 2.

The suspended solids from wastewater are removed

by sedimentation (2) and adhesion on the support

medium (3), the principal mechanisms at the begin-

ning, with adhesion facilitated over time by the

reduction of porosity and formation of nuclei. These

are constituted by the biofilm (8), solids released by

substrate wear (9) and precipitates (4). With reduction

in the pore diameter between the grains of the support

material, they may be smaller than that of SS, favoring

filtration by simple sieving mechanism (1).

The organic solids, originally suspended (and

converted to dissolved solids, after hydrolysis),

together with the original dissolved solids, contribute

to pore obstruction via their degradation by the

microbiota present, with generation of gases and

energy used for microbial growth and reproduction,

increasing the biofilm thickness (8). If the gases (7) are

trapped between the solids retained in the interstitial

space, they also promote a reduction in the drainable

porosity.

Redox potential, pH conditions and the constitution

of the support material can either make the environ-

ment conducive to precipitation (4) or contribute to the

wear of the substrate (9). If the deposited debris

contributes to obstruction, the reduction in granulom-

etry of the grains that make up the support medium

also contributes to the phenomenon by the reduction in

the pore volume of the medium.

The influence of plants (5 and 6) is controversial. In

the image, the occupation of the porous space by the

presence of roots, and the contribution of the vegetal

solids are presented, but other mechanisms are

involved. The role of plants and the other factors

identified as contributing to the clogging process are

detailed in the following items.

2.2.1 Suspended solids in the wastewater

The first hypothesis that can be formulated is that the

suspended solids (SS) from the applied wastewater are

the main cause of pore obstruction. Several authors,

including Platzer and Mauch (1997), Winter and

Goetz (2003) and Zhao et al. (2009) stated that

clogging is related to the characteristics of the influent,

therefore the higher the SS load applied the shorter the

life of the system.

Xu et al. (2013) and Knowles et al. (2011) analyzed

several units and verified that high hydraulic loads

associated with high organic loads are those that most

influence clogging. On the other hand, Hua et al.

(2010) observed that the dimensions of pores are much

larger than those of the solids contained in the influent,

concluding that not only did suspended materials

block the pores, but also dissolved solids and those

from other sources. The authors also concluded that

retention in the porous medium is not only by filtration

(sieving mechanism), since if that were the case the

particles would emerge in the effluent, influencing the

adhesion and sedimentation mechanisms in the

restriction of the porous volume, as well as formation

of the biofilm.

Fig. 2 Factors participating

in clogging

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Hua et al. (2010) and Knowles et al. (2011)

mentioned that the filtration capacity increases with

progressive attachment of suspended solids to the

support medium, thus reducing the hydraulic conduc-

tivity of the medium. Nuclei are then formed, with

greater attraction and adhesion of particles, restricting

the passage of liquid and favoring more contact

between particles. However, according to Xie et al.

(2010), there is a more pronounced decrease in void

volume due to the suspended solids accumulation at

the beginning of the operational period, being gradual

and continuous in later stages, as suggested by Kadlec

and Wallace (2009).

If it is not only the suspended solids that contribute,

the explanation may be the suspended organic fraction

resulting from the wastewater applied to the CW,

because it is known to occupy large volumes given the

smaller specific mass. Tanner and Sukias (1995), for

example, observed a higher porosity in CW that

received lower organic load. In continuation of the

study, Tanner et al. (1998a) found that volatile solids

accumulation rates tended to decrease with the

maturation time of CW when there is a more active

biomass, with higher degradation rates of the organic

material. There was greater accumulation of volatile

solids (VS) in the first 2 years than in the next 3 years,

justifying why the hydraulic conditions stabilized in

the units evaluated during this period. The authors also

verified that the rates of organic material accumulation

(1.3–3.0 kg VS m-2 year-1) during 5 years of mon-

itoring were higher than the load applied annually

(0.4–1.6 kg VS m-2 year-1), suggesting that other

factors such as the contribution of plant solids and

biofilm formation also contributed to the phenomenon.

Tanner et al. (1998a) attributed the greater accu-

mulation of solids in the first years to the possible

decrease in HRT, with a loss in solids retention, which

may imply a possible loss of efficiency (Tanner et al.

1998b). Caselles-Osorio et al. (2007) also attributed

the changes in tendency for solids accumulation over

time to the change of their characteristics, including

specific mass and content of the labile and inert

fractions. Because wastewater contains complex mix-

tures of organic material of different types, chemical

structural chain sizes, dissolved or particulate condi-

tions, easily biodegradable and recalcitrant, as well as

inert compounds (Levine et al. 1991; Kadlec 2003),

temporal changes are possibly due to changes in the

percentages of each of these.

According to Blazejeski and Murat-Blazejewska

(1997), clogging is a complex phenomenon influenced

by different types of solids. Fu et al. (2013) observed

that of the organic matter present in the bed, the

fraction that most contributed to clogging was the

labile organic matter (active organic matter), which

constituted only about 30% of the total organic matter,

and the Fulvic acids. Under anaerobic conditions,

Fulvic acids accumulate and are not converted to

humic acids. Nguyen (2000) observed that 90% of the

organic material present in the solids evaluated by

Tanner et al. (1998a) were stable (recalcitrant), i.e.,

difficult to break down and probably consisted of

lignin, cellulose and humic substances. The same

author verified that 63–96% of carbon present in the

clog-forming material was composed of the humic,

humic acid and fulvic acid fractions. Caselles-Osorio

et al. (2007) observed degradation of only 10% of the

COD of the obstructing material in a period of

20 days, reinforcing the hypothesis of recalcitrance

of a major portion of the accumulated solids.

2.2.2 Biofilm

Water and nutrients are the major limiting factors for

the growth of biological solids that grow in a dispersed

way in the pores or as a biofilm attached to the medium

(Suliman et al. 2006). Because these inputs are

abundantly present in CW, the growth of microorgan-

isms organized in communities adhering or not to the

support material is a potential factor for pore obstruc-

tion. This was indicated in the study carried out by

Caselles-Osorio and Garcıa (2006), who compared the

hydrodynamic conditions in CW to which a solution of

more labile organic material (glucose) was applied and

in another that received a solution of more complex

organic material (starch). The hydraulic conductivity

in the porous medium near the entrance of the HSSF-

CW was lower in the unit that received glucose, with

no difference near the outlets of the systems. The

authors concluded that the main cause of pore

obstruction at the inlet end of the HSSF-CW that

received the glucose solution would have been the

more intense biofilm formation. Thus, while SS would

contribute directly to clogging, organic dissolved

solids and hydrolyzed organic suspensions would

contribute indirectly through microbial growth (De la

Varga et al. 2013).

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Despite this evidence, Langergraber et al. (2003)

and Winter and Goetz (2003) considered that the SS

load is still the most important factor for clogging, and

that biomass growth would have a smaller effect than

the accumulation of SS fromwastewater. According to

Zhao et al. (2009), suspended solids from the

wastewater contribute to faster clogging of the small

pores, whereas formation of the biofilm would have a

more long-term effect. The same authors studied VF-

CW and verified that when applying synthetic

wastewater containing labile dissolved organic mate-

rial, a more homogeneous distribution of the obstruc-

tion along the depth of the porous medium was

observed, while when applying the suspension con-

taining particulate organic material the obstruction

was restricted to the vicinity of the surface. Biofilm

formation may also lead to increased SS retention in

applied wastewater, since according to Vandevivere

and Baveye (1992) and Knowles et al. (2011), under

saturated conditions filamentous colonies are formed,

which are more efficient in the retention of organic and

inorganic solids.

According to Wang et al. (2010), in addition to

microorganisms the production of exopolymers con-

tributes to the restriction of wastewater movement in

the CW. This hypothesis is corroborated by Okabe

et al. (1998) and Herbert-Guillo et al. (2000), because

according to them the biofilm develops a gelatinous

structure of extracellular polymers produced by bac-

teria, which is resistant to shear. Christensen and

Characklis (1999) argued that both the accumulated

sludge and the biofilm present in the substrate and on

the plants form a mixture of organic and inorganic

substances, potentially clogging the porous media.

According to Yan et al. (2008), cited by Fu et al.

(2013), this gelatinous structure is formed when the

organic matter content in the medium exceeds 5%,

constituting about 90% of the biofilm according to

Flemming andWingender (2010), where the rest of the

dry mass is composed of microorganisms.

Christensen and Characklis (1999) reported that the

roughness and viscosity of the biofilm coating may

also act to resist the movement of wastewater through

the bed interstices. Kadlec and Watson (1993)

observed that formation of the viscous gel, observed

mainly in the first quarter of the HSSF-CW, reduced

the porous spaces in the bed by half. However, a report

by the USEPA (1993) indicated that after 2 years of

system operation, the presence of gelatinous

substances in the medium of the clogged material in

the CW evaluated by Kadlec and Watson (1993) was

no longer identified. After aging of the CW, the

surface area and the complex structure of the biofilm

diminish, and its continuous growth no longer implies

a great reduction in the hydraulic conductivity of the

medium. According to Cunningham et al. (1991),

Okabe et al. (1998) and Suliman et al. (2006), there is a

certain balance between decay and bacterial growth

and limits on the size of the attached microbial

community, due to difficulties in the transfer of

nutrients from the periphery in contact with the

wastewater to the center of the biofilm.

Samso and Garcıa (2014) tried to describe clogging

according to the ‘‘Cartridge theory’’, in which there is

greater biofilm formation near the entrance to the CW

at the beginning of operation. The metabolism ends up

generating a great accumulation of inorganic solids,

which causes the microbial active zone to be displaced

to a location downstream where there is a greater

transformation of the organic material. New accumu-

lation of inert material occurs and there is another

change in the position of the heterotrophic microbiota.

With regards to temperature, higher temperature

may favor higher enzymatic activity and thus degra-

dation of the organic material (Bihan and Lessard

2000), however temperature also favors higher rates of

biofilm growth (Platzer and Mauch 1997; Zhao et al.

2009). In the balance between degradation rate and

microbial growth, considering the longer time it takes

for microorganisms to occupy the porous space

compared to the SS, Zhao et al. (2009) and other

authors considered that the temperature rise tends to

reduce obstruction more than the contrary.

Seifert and Engesgaard (2007) studied the effect of

microbial growth on stone filters after inoculation with

sewage, verifying restrictions in water movement

though the columns. However, the authors were not

able to explain how a stone filter that did not receive

sewage, but only clean water, also had a reduction in

its hydraulic conductivity. Thus, it is implied that there

are also other factors contributing to the reduction in

available porosity.

2.2.3 Precipitates and gases

In sewerage systems or in wastewater treatment plants,

the formation of CaCO3 precipitates can contribute

considerably to pore obstruction, as well as calcium

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precipitation in a reaction with silicon present in the

wastewater (Blazejeski and Murat-Blazejewska

1997).

Coppola et al. (2004) cited the precipitation and

deposition of carbonates, phosphates, iron and alu-

minum hydroxides and calcium and magnesium

oxides as important factors in the process of pore

space obstruction. Precipitates formed with carbonate

and constituents of leachates from sanitary landfills,

when flowing through porous medium composed of

blast furnace slag can obstruct the porous spaces,

although Rowe et al. (2000) considered this to be little

significant. Other precipitated salts may also have an

influence on clogging, such as those formed with

heavy metals and phosphorus (Vymazal et al. 1998),

iron and especially iron sulfides, as reported byWinter

and Goetz (2003) and Marshall (2009).

In the case of beds containing reactive support

media, such as blast furnace slag, pH is proportional to

the formation of precipitates (Sakadevan and Bavor

1998; Korkusuz et al. 2005). According to Khadhraoui

et al. (2002) and Suliman et al. (2006), the use of

substrates such as blast furnace slag and crushed

seashells can improve the system efficiency due to the

formation of phosphorus precipitates (calcium phos-

phate) and its removal, however it contributes to rapid

pore obstruction, also forming a retention nucleus of

other solids, enhancing the clogging process.

According to Kadlec and Wallace (2009), in the

short term clogging would be more related to devel-

opment of the root system and biomass, and in the long

term to deposition of solids in mineral suspension and

the formation of insoluble chemical precipitates.

Furthermore, there would be the possibility that

‘‘trapped’’ gases in the pores, such as H2S, could also

hinder the permeability of water in CW (Dillon et al.

2001; Guofen et al. 2010).

2.2.4 Wear of the filter material

Kadlec andWatson (1993), as well as Reed and Brown

(1992), found that about 80% of the fine particles

present in the beds are inorganic. Corroborating the

same conclusions, Caselles-Osorio et al. (2007) and

Paoli and von Sperling (2013a) found that only

20–25% of the clogging solids are organic.

Tanner and Sukias (1995), in their discussion on the

origin of HSSF-CW clogging, attributed much of the

inorganic solids present in the beds to external

contributions. In one of the planted CW, a mass of

19.1 kg m-2 of fixed solids (FS) was found, while that

of volatile solids (VS) was 3.85 kg m-2, making up

17% of the total mass (TS), close to the range

(20–25%) obtained by other authors (mentioned

above). In the bed containing the highest mass of

inorganic solids, which was also that which received

the highest organic loads, the authors attributed the

lower VS/TS ratio to the external contribution of soil

(which has a VS/TS ratio between 5 and 20%).

According to the USEPA (1993), part of the

inorganic solids that clog the pores may be from the

time of constructing the beds and planting of the plant

species in the system. Thus, filling the CW with

previously washed substrate, so that the fine material

present does not contribute to clogging of the porous

medium may be a good strategy. Another interesting

point to be analyzed is the result obtained in the work

of Tanner and Sukias (1995), in which it was verified

that the increase in the applied organic load provided

greater accumulation of organic solids. However, this

was not reflected in the proportion of volatile solids in

relation to total solids, indicating that the presence of

inorganics also increased during 5 years of operation.

If the inorganic load was due only to the wastewater, a

well-defined trend could be observed in the VS/TS

ratio, but this did not occur.

When it is found that the great majority of the solids

obstructing the bed are inorganic, an important line of

investigation is needed. Pedescoll et al. (2009) verified

that the inorganic solids associated with clogging had

a mineral constitution similar to that of the filter

material, therefore it is speculated that an important

part of the fine material associated with clogging is

from the substrate itself (support medium), as a result

of the attack of sulfur acids (sulfuric acid and

hydrogen sulfide) or their wear. Problems in biofilm

sampling/extraction with abrasion in the sample, for

biofilm release, could also contribute to release of part

of the filter material, potentially influencing the

proportion of inorganic solids obtained. However,

Matos et al. (2017b) observed that washing of the

support medium for extraction of the adhered solids

was not a significant factor to decrease the VS/TS ratio

of the sampled material. The same authors also found

that 80% of the clogging solids were inorganic, and

that of these 82% had characteristics similar to those of

the support material (blast furnace slag), indicating

that the physical (abrasion, temperature), chemical

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123

(acid reactions, precipitation) and biological wear

(release of acids by microorganisms and plants, root

growth) were factors of great importance in clogging.

Thus, there would be contribution of two forms,

reduction of the porous space by decrease of the

granulometry and occupation by the inorganic solids.

On the other hand, despite the greater mass presence of

the inorganic solids, the organic material has greater

influence on reduction of the void volume of the

porous medium. Matos et al. (2017b) verified that

there were three times more solids, in mass, from the

wear of the support material than organic solids, but

the latter occupied nine times larger pore spaces, given

its lower specific mass and hence greater volume.

Pedescoll et al. (2011b) evaluated the potential

wear of the filter material by means of mechanical

resistance tests (abrasion, impact and durability after

immersion in solution) of the material and with the

application of water. The authors considered the

hypothesis that crushed debris may be the main factor

for bed obstruction due to its resistance to weathering.

However, the water test conducted by the authors was

not very conclusive, since the characteristics of the

sanitary sewage are different from those of water,

especially with regard to the ability to attack the

material, due to the presence of sulfides and because

the pH is more conducive to rock wear and dissolution.

2.2.5 The role of plants in clogging of CW

A recurrent discussion in papers on the subject of

constructed wetlands is related to the role of cultivated

plants in the clogging process of a CW. Authors such

as Tanner et al. (1998a), Knowles et al. (2010) and

Pedescoll et al. (2011a) stated that the presence of

plants contributes to the clogging of CW. In planted

units, notably in those in which cutting of the plant

shoots is not performed properly during the correct

periods, there is senescence of the plant with the fall of

leaves and death of other components (Tanner et al.

1998a). This leads to accumulation of material on the

surface and slight translocation of organic solids to

greater depths. Kadlec and Wallace (2009) stated that

of this organic material, 5–15% is recalcitrant, there-

fore remaining in the bed and contributing to obstruc-

tion of the voids. Another possible justification for

increased clogging in planted CW is the growth of

roots and rhizomes, occupying the porous space and

thus contributing to the restriction of wastewater

passage (Blazejeski and Murat-Blazejewska 1997).

Pedescoll et al. (2011a) and Paoli and von Sperling

(2013a), among other authors, observed lower

hydraulic conductivity in planted units, as compared

to unplanted units, which would indicate that planted

CW are more subject to clogging. According to

Pedescoll et al. (2011a), after 3 years the roots would

constitute 35–70% of the material that occupied the

porous spaces. According to Knowles et al. (2011), it

has been demonstrated that the presence of roots

creates greater resistance to flow, with preferential

paths where there is no influence of roots.

Tanner et al. (1998a) stated that the plants con-

tribute about twice as much to the accumulation of

material in the medium as the wastewater itself, a

conclusion obtained by analyzing the carbon/nitrogen

(C/N) ratio of solids accumulated in the pores of

planted (16.5) and unplanted units (11.0). Because it is

known that the C/N ratio found in dead plant tissue is

20, it is suggested that there is a significant participa-

tion of accumulated solids in planted CW (Tanner and

Sukias 1995). However, when comparing the same

units with regards to accumulation of solids at

different depths, in some cases they found lower

values close to the impermeable layer in the planted

beds. Near the surface, possibly in function of the

coverage provided by the plant shoots, reductions in

temperature variations may have resulted in lower

organic material degradation rates and subsequent

high material accumulation (Brix 1994). Similarly,

Pedescoll et al. (2009) also observed greater presence

of plant solids in the unsaturated zone and in the first

10 cm of the saturated zone of the CW, demonstrating

their low mobility in the medium. According to

Knowles et al. (2010) and Xie et al. (2010), this

condition of higher surface clogging associated with

the presence of plants had a great influence on the

obtained hydraulic conductivity value. Baptestini et al.

(2017), on the other hand, did not observe a significant

difference between cultivated and uncultivated HSSF-

CW, nor the effect of different species on the hydraulic

conductivity value.

Authors, including Brix (1997) and Brasil and

Matos (2008), argued that roots favor flow through the

bed, counteracting the effects of clogging due to

expansion of the local porous space due to the

‘‘swelling’’ (bulking effect of volume expansion due

to penetration of roots in the pore spaces) resultant

94 Rev Environ Sci Biotechnol (2018) 17:87–107

123

from root growth. According to Stottmeister et al.

(2003), Whitney et al. (2003) and Turon et al. (2009),

the death and subsequent degradation of roots and

rhizomes by microbial action favors the formation of

secondary channels, which would facilitate wastewa-

ter flow. Thus, the drainable or effective porosity,

which are associated with the interconnected pores

through which the liquid flows, could be larger than in

units without vegetation.

Furthermore, the release of exudates and the supply

of oxygen via aerenchyma to the root zone of plants

creates an environment conducive to greater microbial

diversity, stimulating the degradation of accumulated

organic material and favoring faster unclogging of the

pores (Brix 1997). If there are conditions suitable for

microbiota proliferation, the presence of roots also

allows for increased surface area of the porous

medium for fixation of microorganisms, favoring the

formation of biofilm (Zhao et al. 2009).

According to Knowles et al. (2011), some authors

state that evapotranspiration and higher retention of

wastewater in the system result in an increase in HRT,

being sufficient to compensate for any contribution

that plants may have on the hydraulic conductivity loss

of the porous medium of CW. In summary, if on the

one hand there is greater contribution with the input of

organic material to the superficial layers of the porous

medium, which can be reduced with adequate main-

tenance (cutting of the shoots and removal of plant

material from the CW at appropriate times, and the

removal of invasive species), several factors can be

indicated as beneficial effects of the presence of plants

in relation to unclogging of the porous medium

(Knowles et al. 2010). Together these mechanisms

could justify greater attenuation of clogging and

resistance to surface flow in planted HSSF-CW in

operation in Spain, as reported by Gari et al. (2012).

This can explain the similar or higher HRT in planted

beds compared to non-cultivated CW, measured using

tracers, even though there is lower hydraulic conduc-

tivity (Paoli and von Sperling 2013a, b) or higher

surface solids concentration (Tanner and Sukias

1995). Matos et al. (2015) evaluated the same CW as

Paoli and von Sperling (2013a), and after 5 years

found a higher HRT in the planted unit, indicating that

the roots may have been able to greater attenuate

clogging when in a more advanced stage.

Fu et al. (2004), cited by Zhao et al. (2009),

observed slightly higher hydraulic conductivity in the

upper layers of a VSSF-CW compared to measure-

ments in the central layers of the porous medium,

which were attributed to the presence of plants.

According to Brix (1994), Molle et al. (2006) and

Knowles et al. (2011), in vertical units there is also an

additional mechanism for attenuation of the resistance

to flow, known as ‘‘lever arm’’. The wind creates an

oscillatory movement of the plants, creating a distur-

bance in the medium, with opening of cracks in the

layers where there is accumulation of solids, benefit-

ing flow in the VSSF-CW.

Fu et al. (2013) observed that clogging first

appeared in the unplanted CW, compared with the

planted units, and that this process was different

according to the plant species cultivated in the system.

The C. indica species provided conditions for longer

useful life compared to C. anternofolius. It was also

possible to observe that the phenomenon of pore

obstruction in planted CW is more complex and has

more involved factors than in those without vegeta-

tion. In the unplanted CW, the labile organic material

is the main component of clogging, due to the lower

capacity of the system to provide oxygen and the

relative low diversity in the heterotrophic microbial

community for this degradation to occur.

Wang et al. (2008), cited by Fu et al. (2013),

showed that depending on the plant species cultivated

in the CW, an increase of 3–44% in the porosity of the

medium is possible. Thus, the results suggest that

different plant species guarantee different responses in

the sense of attenuation of the clogging phenomena,

mainly due to the ‘‘swelling’’ effect and influence of

the microbial community.

Munoz et al. (2006) observed that the presence of

plants and the aeration they generate provided greater

preservation of the initial porosity in CW, also

contributing as a barrier against obstruction caused

by the freezing of the water in the layers of the porous

medium, due to the already discussed effect of reduced

thermal fluctuations. The authors further argued that

plants can both provide sedimentation and resuspen-

sion of accumulated solids in the system, reducing

clogging in the porous medium.

According to Hua et al. (2014), plants may

influence clogging in different ways. The authors

observing that at the beginning the roots restrict

movement of wastewater in the porous medium,

however over time there is an inversion of this

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123

tendency, resulting in increased porosity and the

favoring of flow.

3 Methods for identification of clogging

In order to minimize clogging and to allow for

adequate techniques of system maintenance, it is

important to know the main factors that lead to the

occurrence of clogging and to identify the degree of

obstruction in the porous medium. However, because

this is a phenomenon that occurs in the subsurface and

has several associated factors, it is difficult to identify

it and there is no consensus on which method could

best describe the condition of the CW (Nivala and

Rousseau 2009; Morris and Knowles 2011). For this

reason, often times it is observed that the bed is

blocked by observation of surface flow when it is

already in a critical condition, reducing opportunities

for remediation of the unit.

Methodologies used in filters are not reproducible

in CW, depending on the characteristics of the reactor.

The low cohesive substrate prevents the withdrawal of

undisturbed samples for laboratory analysis, while the

long HRT and exposure to atmospheric conditions

hamper the measuring of drainable porosity in the field

(Pedescoll et al. 2011c). Thus, in situ methods are

commonly employed, which employ the extraction

and quantification of solids, or indirect methodologies

to evaluate the pore obstruction using hydrodynamic

(hydraulic conductivity and salt tracer) and geophys-

ical properties (georadar and others). Table 1 summa-

rizes some of the methods, including solids sampling

and hydraulic conductivity (permeameters) and others

still incipient, presenting their advantages and

disadvantages.

As discussed in the item referring to the role of

plants, the different responses obtained depend on the

methodology selected. The use of permeameters in

fields, for example, is strongly influenced by the first

layers, where given the senescence, low translocation

of organic material and lower thermal amplitude, as

discussed, there may be greater accumulation of solids

in the planted unit. Furthermore, if application of the

liquid occurs over a section where the root and

rhizome structures are present, it is expected that there

will be greater difficulty in flow. At greater depth,

better conditions may be found in the planted CW.

On the other hand, the introduction of tracer

substances for simulating the movement of liquid

tends to better reproduce the conditions of the units.

However, the HRT is influenced by evapotranspira-

tion, characteristics of the tracer and only allows for

describing the unit as a whole, and not its individual

compartments (Chazarenc et al. 2003, 2010).

Sampling of solids is the method that allows for

direct inference of the CW conditions. However, it has

the disadvantages of being invasive and not describing

the actual situation. The quantitative analysis of solids

masks the differences in concentration as a function of

depth and volume of each type of accumulated

material. As discussed, volatile solids are usually

found in lower concentration, but have a lower specific

mass, occupying more space. Thus, it is necessary to

develop different methods, such as those described in

Table 1, or to obtain agreement among responses from

different methodologies, to better characterize the

degree of clogging, as suggested by Morris and

Knowles (2011).

Matos et al. (2016) and Matos et al. (2017a) were

successful, respectively, in the use of ground pene-

trating radar (GPR or georadar), with the development

of equations, and to estimate porosity of the cross

sections in the use of Planted Fixed Reactor (PFRs).

The results showed characterization closer to the

actual conditions of the systems, as opposed to

conventional methodologies for measuring hydraulic

conductivity (Matos et al. 2017a) and analysis of the

solids concentration (Matos 2015).

4 Forms of attenuation of the clogging process

Amado et al. (2012) and Pozo-Morales et al. (2013)

associated the clogging phenomenon with the porous

medium conditions in the CW, and in a more reducing

environment (lower redox potential or negative pE),

the degradation rate would be lower since the

metabolism of anaerobic microorganisms is slower

than aerobic microorganisms, and therefore there is a

greater concentration of solids for clogging (Mata-

moros and Bayona 2006; Hijosa-Valsero et al. 2010).

It can therefore be inferred that a shallower wet depth

may favor greater removal of organic material, and

therefore less clogging of the porous medium (Huang

et al. 2004; Garcıa et al. 2005; Song et al. 2009). For

this reason, Pozo-Morales et al. (2013) proposed a new

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Table 1 Summary of the applicability and advantages and disadvantages associated with the available methods for evaluation and

obtaining the degree of clogging in CW. Source: aPlatzer and Mauch (1997), bCETESB (1999), cChazarenc et al. (2003),dLangergraber et al. (2003), eSuliman et al. (2006), fCaselles-Osorio et al. (2007), gCooper et al. (2008), hGiraldi and Ianelli (2009),iKadlec and Wallace (2009), jXie et al. (2010), kKnowles et al. (2010), lPedescoll et al. (2011c), mKnowles et al. (2011), nMorris and

Knowles (2011), oNivala et al. (2012), pTapias et al. (2013), qBarreto et al. (2015), rMatos et al. (2015), sMatos et al. (2016), tMatos

et al. (2017a), uMatos et al. (2017b) and vMiranda et al. (2017)

Method Applicability Advantages Disadvantages

Bench permeameter

(variable or constant

load) (references: d, k,

l, n)

Measurement of hydraulic

conductivity in cohesive soils

and substrates

Laboratory test

Controlled measurement in

the laboratory

Better conditions for

comparison of the results

It is not possible to remove

undisturbed samples for analysis

Use on pebble substrates is not

recommended

Constant head method

(CHM) (references: i,

k, n, t)

Measurement of hydraulic

conductivity in soils, filters and

CW

Field test

Standard method for CW More difficult execution compared

with the FHM

Invasive

Better simulates conditions in VF-

CW

It is not recommended to compare

results in beds with different

configurations and operating

conditions

Responses strongly influenced by the

top layers

Does not always present good

correlation with the solids sample

Falling head method

(FHM) (references: i,

k, n, t)

Measurement of hydraulic

conductivity in soils, filters and

CW

Field test

Easier execution

Although simpler, presents

results consistent with the

CHM

Better simulates conditions in VF-

CW

Invasive

It is not recommended to compare

results in beds with different

configurations and operating

conditions

Greater variability of results obtained

in relation to the CHM

Responses strongly influenced by the

top layers

Does not always present good

correlation with the solids sample

Planted fixed reactor

(PFR) (references: q, t)

Measurement of hydraulic

conductivity removable baskets

Baskets submitted to field

conditions, but the analysis is

performed in the laboratory

Better simulation of

hydrodynamics in HSSF-

CW

Non-invasive

Requires the installation of baskets

when constructing the CW

Requires removal and transportation

for laboratory analysis

Slug test (references: n) Measurement of hydraulic

conductivity in soils

Field test

Invasive

More indicated for VF-CW

More sophisticated method

Infiltration ring

(references: n)

Measurement of hydraulic

conductivity in soils

Field test

Simple execution Invasive

More indicated for VF-CW

Pumping test

Steady state test

Unlined Auger Hole

(references: n)

Measurement of hydraulic

conductivity in saturated

environments (aquifers)

Field test

Sophisticated execution

Labor intensive

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123

Table 1 continued

Method Applicability Advantages Disadvantages

Modified Cube Method

(references: n)

Measurement of hydraulic

conductivity in soils and

cohesive media

Laboratory test

Measurement of hydraulic

conductivity on all three

axes, including horizontal

Difficult to remove undisturbed

samples

Sampling of solids

(references: f, s, v)

Quantification of the solids

concentration in filters and CW

Field test

Simple execution

Direct method of obtaining

the degree of clogging

Can be used for comparison

between different units

Requires interruption of the

wastewater supply to treatment

Requires draining of the system

Subject to losses by washing of the

solids

The specific mass appears to be more

important than the mass itself

Very invasive to the bed

Test with tracers

(references: c, o, r)

Acquisition of the HRT,

volumetric efficiency and other

hydrodynamic variables of

reactors

Field test

Responses of the liquid flow

conditions in the reactor

Suitable for both vertical

and horizontal units

The results can be influenced by the

type of tracer because there is no

ideal substance

Problems with toxicity, preferential

vertical movement (difference in

density) and low recovery

Depending on the selected substance,

may require large and specialized

equipment

Responses of the system as a whole

and not conditions of each section

Capacitance probe

(references: a, h, n)

Determination of the water

content in the medium

Association of the water content

with the organic material

content

Field test

Non-invasive method

Promising method

More than 95% of the organic solids

are water, impairing analysis in

permanently flooded reactors

More suitable for intermittent

systems, such as VF-CW

Requires sophisticated equipment

Time- domain

reflectometry (TDR)

(references: a, h, n)

Determination of the water

content in the medium

Association of the water content

with the organic material

content

Field test

Non-invasive method

Promising method

More than 95% of the organic solids

are water, impairing analysis in

permanently flooded reactors

More suitable for intermittent

systems, such as VF-CW

Requires sophisticated equipment

Electrical resistivity (ER)

(references: b, n, p)

Mapping of metallic and

inorganic contamination

Field test

Non-invasive method

Promising method

Less indicated for characterization of

obstruction by organic solids

Suitable for very deep assessments

(roughly 100 m), a condition not

found in CW

Requires sophisticated equipment

Magnetic resonance

(references: b, n)

Contamination mapping

Field test

Non-invasive method

Promising method

Sophisticated equipment

Georadar (GPR)

(references: b, n, g, s)

Mapping of organic and

inorganic contamination

Field test

Non-invasive method

Promising method

Identification of obstructed

and open pores

Sophisticated equipment

Difficult to apply in planted

environments

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configuration for the CW, with openings for entry of

air in the inlet end and a differentiated size distribution

of the support material. Thus, there would be greater

gas exchange and aeration, providing more aerobic

conditions to the system.

With regards to granulometry of the support

material, the larger the particle size of the substrate

used to fill the CW the larger the pore volume, and

therefore the time until appearance of surface flow

should be higher (Zhao et al. 2004; Hua et al. 2010).

Additionally, with a larger particle size there is a

smaller surface area, and therefore the amount of

biofilm adhered is less. Thus, clogging of the porous

medium takes longer, but the effluent may be of lower

quality since there is reduced filtration and microbial

degradation of the organic matter (decrease in the

removal efficiency). This means there should be

equilibrium between the filtering effect and the speed

of clogging of the porous medium. Griffin et al.

(2008), in a study involving several HSSF-CW in

England, in which the substrate presented granulom-

etry between 6.0 and 12.0 mm, did not verify reduced

removal of pollutants when compared with CW filled

with smaller-diameter material. More recent construc-

tion of CW includes the placement of larger diameter

substrates near the entrance, to avoid premature

clogging and to facilitate the distribution of wastew-

ater in the HSSF-CW (Hudcova et al. 2013), followed

by smaller size particles to favor the physical and

biochemical phenomena, mainly those associated with

removal of pollutants. Hua et al. (2010) also observed

that materials of higher granulometry provide better

distribution of accumulated solids in the porous

medium, with greater loss of porosity at greater

distances in HSSF-CW.

According to Blazejeski and Murat-Blazejewska

(1997), the larger the pore volume of the support

medium the longer the useful life of the bed. To assure

a greater drainable porosity, the diameters distribution

is as important as the size range of the substrate

particles that fill the CW, according to Mancl and

Rector (1999), cited by Suliman et al. (2006).

According to the authors, with a coefficient of

uniformity (CU) lower than 3.0, i.e., poorly graded,

the bed can operate for a long time without the need for

rest periods. With substrates of smaller granulometry

or well graded, there is greater contact between the

particles, thus reducing the voids between them.

Similarly, the shape of the grains composing the

substrate may also imply less drainable porosity, as

discussed by Kadlec and Knight (1996), Hyanova

et al. (2006) and Knowles et al. (2011). The authors

state that non-spherical or angular particles accelerate

the accumulation of solids, due to the greater possi-

bility of approximation of the particles, with conse-

quent decrease in size of the pores.

The type of material making up the porous medium

has also been cited as influencing clogging. Albu-

querque et al. (2010), for example, when using

expanded clay that has a high specific surface area

and porosity as a CW medium, verified a lower head

loss in the system than in that of crushed stone, which

is an indication of greater permeability of the alterna-

tive medium. Suliman et al. (2006) also obtained lower

loss of drainable porosity in expanded clay than in the

CW filled with crushed seashells. The authors

attributed these results to the fact that expanded clay

naturally exhibits greater porosity and the more

reactive characteristics of crushed seashells, which

result in precipitate formation and as a consequence

more clogging of the pores in the medium.

Hudcova et al. (2007) suggested that in large CW

the wastewater has more than one point of entry, since

according to Vymazal (2003), the largest removals and

clogging of the porous medium occur at the entrance.

Large length/width (L/W) ratios lead to flows more

indicative of the plug-flow pattern, which would favor

the highest removal efficiencies of pollutants accord-

ing to first-order kinetics. On the other hand, beds with

this more elongated configuration are affected earlier

by the clogging phenomena. This is because the

smaller the cross-sectional area, the greater the load

applied per square meter of cross section, and

consequently the greater the head loss. For this reason,

CW built in the Czech Republic were divided into

smaller units arranged in parallel, in order to minimize

the problem of early clogging of the porous medium.

Another way to change the method of applying

wastewater to previously installed units in order to

extend the useful life of the system is to have the

wastewater inlet along the length of the bed (larger

dimension) rather than its width (smaller dimension),

because this way there is a greater cross-sectional area

to receive the solids load and influent flow.

In order to alleviate the problem of clogging in CW,

it is possible to reduce the solids load by means of

initial primary or secondary treatment, as observed by

Caselles-Osorio et al. (2007), or restrict the applied

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123

load. The authors found that 60–80% of particulate

COD from domestic sewage had dimensions larger

than 1.2 lm and only 12–34% were in the dissolved

form (\ 0.2 lm). Pozo-Morales et al. (2013) reported

that clogging problems are reduced in horizontal flow

systems that receive loads less than 20 g m-2 day-1

of COD and 5 g m-2 day-1 of SS. Garcıa et al. (2004;

2005) recommend the application of a maximum of

6 g m-2 day-1 of BOD to avoid a significant decrease

in the removal efficiency of organic matter and

ammoniacal nitrogen in HSSF-CW. These values

were adopted by Gari et al. (2012), without the

observation of surface flow and efficiency loss during

a 4-year operational period. However, the hydraulic

conductivity in the CW located in the coldest

environment fell to 36 m day-1, close to what is

considered by de la Varga et al. (2013) as critical

(20 m day-1). Wojciechowska et al. (2010) cited the

values recommended by the USEPA (2000) of

5.4 g m-2 day-1 of TSS and 15–20 g m-2 day-1 of

COD as appropriate loading rates in HSSF-CW. It is

observed that there are few reports of the rate to be

applied, so as not to cause clogging of CW due to

differences between the flow direction, granulometry

and type of substrate used, wastewater composition,

period of occurrence of the phenomenon and the local

climatic conditions. Due to these different variables, in

addition to the influence of other factors in clogging, it

becomes complicated to propose recommended

values.

Pretreatment may be important, especially if the

organic material present in wastewater has large

chemical chains and structural complexity, thus

requiring prior hydrolysis. In this case, there would

be time for entry of simpler compounds into the

microbial cells, with degradation and transformation

of the compound in the CW units. For this reason,

Alvarez et al. (2008), Ruız et al. (2010) and Pedescoll

et al. (2011a) utilized and evaluated the use of

anaerobic reactors upstream of the CW, in order to

increase the useful life of the system. De la Varga et al.

(2013) observed accumulation of solids

(1.56 kg m-2 year-1 of SS) in the system lower than

those obtained by Tanner et al. (1998a) and Caselles-

Osorio et al. (2007), which were in the range between

1.2 and 6.8 kg m-2 year-1 of SS when using initial

primary treatment of sewage. The same authors stated

that in general it is not possible to correlate the applied

solids load with the accumulation of solids in the bed,

indicating that there are other factors involved.

Similarly, the hydraulic conductivity was higher than

that measured in the HSSF-CW monitored by Case-

lles-Osorio et al. (2007), with no surface runoff after

5 years of operation. However, according to Tanner

et al. (1998a), it is not just any pretreatment that can

increase the useful life of the system. According to the

authors, solids sedimentation units, such as settlers,

are preferable to stabilization ponds because of the

particulate BOD (algae) resulting from the latter.

Together with the preventive measures to reduce

the input of organic material and the question of CW

configuration, there are also recommended values in

literature for the load applied to the cross-sectional

area of the unit. TVA (1993), cited by Nivala et al.

(2012), suggests values of 244–488 g m-2 day-1 of

BOD, while Kadlec andWallace (2009) stated that it is

prudent not to exceed 250 g m-2 day-1 of BOD for a

bed with diameter D10 (that allows passage of 10% of

the sample mass) greater than 4 mm. However, as

stated by Nivala et al. (2012), this ‘‘rule’’ has

limitations because it assumes that most wastewater

solids are organic and do not consider precipitates and

recalcitrant material.

In the case of intermittent application with suffi-

cient periods of rest, the useful life of the system can

be increased by degradation of the volatile solids

present (Zhao et al. 2009). Bancole et al. (2003)

verified that the application of lower flow rates

allowed for more uniform development of biomass

in filters, delaying clogging. However, because other

factors appear to be involved and the remaining

organic material is predominantly recalcitrant, this

increase in the useful life may be more important at the

beginning of system operation. If the feeding regime

cannot be intermittent, it is recommended that distri-

bution be as uniform as possible so that there is no

excessive accumulation of solids in certain locations

of the HSSF-CW. Wallace and Knight (2006) suggest

the use of ‘‘T’’ or ‘‘H’’ pipes for this purpose. Shen

et al. (2010) found that the application, in either of

these layouts, provided greater efficiencies and longer

useful life than the conventional HSSF-CW. In

addition to the control of wastewater distribution, it

is also necessary to consider the level of the effluent

collection drain, as well as control of the organic

material accumulated on the bed surface, the invasive

plants and the size or age of the plants cultivated

(Cooper et al. 2005).

100 Rev Environ Sci Biotechnol (2018) 17:87–107

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There are also models in literature that may help

predict when the system will collapse, i.e., it will be

completely clogged. This would permit that arrange-

ments be made to remedy the situation before the

clogging phenomenon could hamper the system

efficiency. Nivala et al. (2012) classified these models

into two groups, the first based only on the influent

solids load, and the other which includes other factors,

such as biofilm formation and chemical precipitation.

However, neither of these models proved to be

adequate on a field scale, since, as was discussed

earlier, many processes are involved and the simpli-

fications and considerations proposed in the models

lead to quality loss in the predictions.

Lancheros et al. (2017) suggest the use of an index,

called Clogging Index. The variable relates to

hydraulic conductivity, nominal hydraulic holding

time and CW length. The lower the value of the index,

the greater the degree of clogging.

5 Clogging remediation

After characterizing the region of the porous medium

as clogged and having knowledge of the main factors

that have led to this, techniques can be used to mitigate

the problem and increase the useful life of the CW

without having to change the medium used in filling of

the system, which it is costly. According to Kadlec and

Wallace (2009), substitution of the medium consti-

tutes about 10–19% of the initial cost of the work, in

addition to the costs of its final disposal. Medium

exchange is one of the suggestions of Xu et al. (2013),

who also recommended the replacement or seasonal

removal of the plants.

After removal of the medium, the material can be

reused, after washing and removal of the clogging

solids, or replaced with a new material. If replacement

is suggested, then the replacement material should

have a larger particle size than the previous one, which

would thus reduce the preferential paths and the short

circuits in the system. In both options there are still

expenditures for the disposal of clogging material,

and, in the case of reuse, costs for the acquisition and

transportation of the new material and transportation

of the medium to the landfill are avoided (Knowles and

Davies 2009; Nivala et al. 2012).

Another method, also costly, is the ex situ and on-

site unclogging of the system, with washing or

application of chemical oxidants in the porous

medium, a technique still under development (Pedes-

coll et al. 2009). Nivala and Rousseau (2009) applied

hydrogen peroxide to the porous medium in an attempt

to unblock the clogged CW. This technique proved to

be effective, however it requires great attention and

safety measures of the operator, and it is also

necessary to remove the accumulated material on the

surface of the bed so that the product reacts with the

interstitial solids. Behrends et al. (2006a) achieved a

50% reduction of the volatile material retained in

porous medium on the laboratory scale, using this

technique associated with the addition of nitrogen

fertilizer. On the other hand, Hanson (2002) and

Behrends et al. (2006b) in the field verified only

displacement of clogging from one stretch to another

downstream. Guofen et al. (2010) evaluated four

different substances (NaOH, HCl, NaClO and deter-

gent), where NaOH and NaClO most efficiently

unclogged the pore space of the CW. The microbiota

recovered after a few weeks, however sodium

hypochlorite was detrimental to the roots of the plants.

Seifert and Engesgaard (2007) inoculated microor-

ganisms in the filters and observed that the hydraulic

conductivity increased after the use of chlorine, a

disinfectant agent. In view of the success of the use of

sodium and chlorine in the tests, it is also necessary to

evaluate if there is a dispersive effect, disrupting

formed flocs and thus facilitating passage of the liquid

through the porous medium.

In an attempt to avoid using chemicals, Wang et al.

(2010) sought to evaluate the efficiency of earthworms

in VSSF-CW, presenting good results, as did Li et al.

(2011). The earthworms act by opening paths and

galleries in the porous media that is clogged, favoring

wastewater flow. Among the previously cited alterna-

tives, this is the least costly, however given the

different conditions of the horizontal flow units, in

which the medium is saturated and therefore unfavor-

able for the survival of the earthworms, success is not

expected in these reactors. For this reason, Davison

et al. (2005), cited by Nivala et al. (2012), verified a

more localized effect, with a dry matter reduction of

56% near the surface of a HSSF-CW.

Behrends et al. (2006a) evaluated the effect of

various techniques, such as fluidization and pumping,

addition of microorganisms, addition of nutrients

(nitrogen) and hydrogen peroxide, with the latter two

being the most effective. The addition of a nutrient

Rev Environ Sci Biotechnol (2018) 17:87–107 101

123

solution to provide adequate N and P ratios in the

clogged mediummay lead to a greater development of

microorganisms, implying a reduction in VS concen-

trations. Using this technique, Miranda et al. (2016)

were able to reduce the concentration of volatile solids

in HSSF-CW by up to 33%.

Another possible form of unclogging the system is

to exert a counter hydraulic pressure to remove the

solid material retained in the pores, a technique known

as backwashing of the porous medium. Backwashing

of a VSSF-CWwas evaluated by Fei et al. (2010), who

stated that it provided increases in the hydraulic

conductivity of the system and removal of COD from

the wastewater under treatment. For HSSF-CW, this

alternative, however, appears to be feasible only for

small units (short length), given the pressure that is

required for backwashing. Baptestini et al. (2016), for

example, did not achieve good results in reversing the

flow direction of HSSF-CW after waiting for the

surface flow to reach 50% of the bed. The authors

believe that the use of the technique in a less advanced

stage of clogging could have provided better results.

Thus, it is important to have good methods of

evaluation of clogging, which allow its detection

before the appearance of surface runoff.

There are also authors who suggested ‘‘rest’’ as a

natural form of depletion of the CW (Xu et al. 2013)

for periods sufficient to degrade the retained organic

material. This period of rest allows for greater

endogenous metabolism and microbial decay (Lev-

erenz et al. 2009). Batchelor and Loots (1997) were

able to reduce the extension of surface flow by resting

the system for 2 weeks. The recommendation to have

rest periods or to perform aeration of the bed seems to

be effective, since, according to Carballeira et al.

(2017), aerobic biodegradation (35% of total) is much

more efficient at removing accumulated solids in

HSSF-CW than anaerobic processes (4% of the total).

Turon et al. (2009) and Xu et al. (2013) recom-

mended techniques for unclogging which included the

inoculation of microorganisms in the system to

promote degradation of organic matter, cutting of

plant species, removal of volatile solids from the

surface and dissolution of precipitates by applying

acidic solutions. There are also suggestions of using

forced aeration in saturated areas of the CW, where

oxygen is injected in order to provide rapid biodegra-

dation of the organic material (Tang et al. 2009).

Labella et al. (2015) observed positive effect of

aeration on removal efficiency and of bed heat in

reducing the concentration of accumulated solids.

Selection of the remediation technique for saturated

CW must be preceded by prior characterization of the

solids constituents accumulated in the pores, since

success is not expected for unclogging of porous

media filled with inert material by simply using

techniques for oxidation of organic material. Addi-

tionally, some techniques such as oxygen injection in

clogged zones may accelerate the formation of

recalcitrant byproducts and iron precipitates (Nivala

et al. 2007) or other compounds. Finally, because CW

is a simple and natural system, and this is one of its

advantages, techniques that are easy to perform and

that interfere as little as possible in the operating

conditions of the CW should be prioritized.

Despite the concern about HSSF-CW clogging,

there are only few studies with long-term monitoring

that demonstrate the problem associated with the

emergence of surface flow. Vera et al. (2011), for

example, found a common oscillation in BOD effluent

concentrations, complying, in almost all periods, the

European legislation regarding the discharge of efflu-

ents into receiving water bodies, in the 8 years of

monitoring of 11 treatment wetlands in Spain, some of

them without post-treatment. In a monitoring for the

same period of time, Aiello et al. (2016) reported that,

despite the signs of clogging, efficiencies remained

unchanged in units installed in Italy. In Brazil, and also

for 8 years, Matos et al. (2018) indicated that there is

no significant difference and no downward trend in

efficiency in planted and non-planted HSSF-CW along

time. Going further, Vymazal (2018) compiled

20-year data from four HSSF-CWs and found that

the observed depositions did not have a significant

effect on the quality of the effluent, nor was there any

change after the change of the support material close

to the inlet end. The results indicate the robustness of

CW and suggest that assessments are needed in longer

periods of time, both to evaluate methods of identi-

fication, prevention and remediation of clogging, as

well as to infer about its effect on wastewater

treatment performance.

6 Final considerations

Although there is much to learn regarding the under-

standing of genesis, form of control and remediation of

102 Rev Environ Sci Biotechnol (2018) 17:87–107

123

clogging, in recent years much useful information has

been made available. With better understanding

regarding the influence of the medium on the speed

of clogging, it is indicated that the material should be

as inert as possible, being less subject to wear and

therefore the release of inorganic solids that contribute

to clogging of the pores of CW. Furthermore, oper-

ational strategies can be implemented in order to allow

for longer system life before it collapses by complete

clogging of the porous medium. The addition of

substances capable of increasing the degradability of

pore-clogging organic material may also be a suit-

able technique to avoid the need for more radical

measures such as complete replacement of the

substrate or subjecting it to washing.

However, as discussed throughout the text, there is

still much to be understood about the phenomenon of

clogging, the effect of each factor, the most favorable

method to evaluate it, and the most efficient technique

to unclog the porous spaces. The monitoring of HSSF-

CW for longer periods of time can help answering

these questions, as well as assisting in verifying the

effect of the phenomenon on the performance of the

units. Research has indicated that CWs are efficient,

complex, and robust units.

Acknowledgements The authors would like to thank the

agencies CAPES, CNPq, FAPEMIG and UNESCO-IHE for

their support to the research, Copasa for providing the site for

the experimental units, CAPES for the scholarship of the first

author and to the Federal University of Minas Gerais.

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