appendix 21 wastewater quality assessment of the ... · wastewater quality assessment of . the...

Appendix 21

Wastewater Quality Assessment of

the Biological Trickling Filter

ENVIROKNOWLEDGE® 3 Dunrobin Street, Dunedin 9013 Phone 03 4776111

www.enviroknowledge.co.nz

ENVIROKNOWLEDGE® and NKnowledge® are registered trademarks.

From: Selva Selvarajah, ENVIROKNOWLEDGE Ltd

To: Wolfgang Kanz, Gisborne District Council

Date: 30 June 2017

Topic: Wastewater quality of the current single BTF

Summary

The GDC BTF is capable of treating COD (550 → 200 g/m3), BOD (250 → 50 g/m3) and total-N

(40 → 24 g/m3) well (with treatment efficiency of 64, 80 and 40% respectively). Much of the

COD and BOD from BTF effluent (>90%) can be removed by solid settling. Given half of the 17

TKN g/m3 in the BTF effluent can be further removed by solid settling, the N treatment

efficiency can be considered as 60% (40 → 15.5 g/m3).

However, the treatment of TP is not efficient with the BTF effluent total-P level exceeding the

influent total-P at times. This was not surprising given the combined complex N and P

assimilative process within the BTF filters and recirculating of the BTF effluent.

Judging by the limited dataset available, the existing BTF was capable of nitrifying well

(biological oxidation of ammoniacal-N to nitrate), which is a feature that may assist in further

treatment of nitrate-N in constructed wetlands by denitrification. However, this requires

optimising current BTF to improve nitrification.

Nitrification in BTF could also be improved by reducing COD and BOD loadings. This can be

achieved by splitting the influent between two parallel BTFs. Despite this available option, it is

sensible to begin optimising the existing BTF to improve N treatment by assimilation and

nitrification of ammoniacal-N. This will be critical to make decision on the need of double BTF

and if adopted, to enable better BTF design and performance of the additional BTF.

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GDC information assessed:

NIWA Wetland trial spreadsheet SP1 which has BTF effluent data from 23 Feb 2016 to 2

August 2016 following solid removal by NIWA conical method

GDC BTF wastewater quality data collected during NIWA wetland trials from 22 May

2013 to 10 August 2016

GDC BTF data spreadsheet for BTF before and after wastewater quality from 6 January

2011 to 27 December 2016

Biological Trickling Filters

Biological trickling filters (BTFs) are well known for their stable aerobic wastewater treatment

processes. The old conventional BTFs contain rocks as filters without active aeration of media.

Wastewater is sprinkled by rotating arms which feed aerated wastewater to rock media.

Wastewater trickling down the media creates biofilms and bio-growth around the media which

act as micro-treatment systems, with concurrent aerobic and anaerobic bacterial activities.

When solid growths are mature, they fall off naturally (sloughing) or by wastewater flushing.

Most BTFs have been designed primarily to treat carbonaceous materials (BOD and COD).

However, BTFs are also capable of treating nitrogen (N) by bacterial assimilation of

ammoniacal-N (NH4-N + NH3-N) into dead bacterial cells (organic-N) and nitrifying

ammoniacal-N into nitrate-N (NO3-N) by biological oxidation process, followed by

denitrification of nitrate-N (NO3-N). Bio-growth on filters provide both aerobic and anaerobic

conditions to promote nitrification and denitrification simultaneously. Of the above processes,

N assimilation is the most predominant N treatment process in BTFs. However, more

nitrification can be promoted by reducing BOD and COD loadings or by splitting wastewater

into two BTFs.

The modern BTFs are sophisticated with active aeration of the media, with media containing

large surface area with greater wettability for bio-growth. For example a 60o crossflow plastic

filters can contain larger specific surface area and void ratio. BTF filters are extensively

researched for their high specific surface area, void ratio, uniformity of bio-growth, filter flies





(which could occur if filters have dry areas), plugging (potential for solids to bridge voids) and

oxygen transfer characteristics.

Purpose of this memo

The main purpose of this memo is to assess all available GDC BTF wastewater data and provide

as much useful information as possible to GDC.

Observations from GDC data:

GDC BTF Specifications

Table 1 contains few basic specifications for the existing single BTF. Judging by the wetting

rate of 39 m3/m2/d and the current high summer average BOD loading of 0.82 kg/m3/d this

BTF can be classified as a high rate trickling filter. Such BTFs can be expected to reduce BOD

by 65-85%. The polypropylene filter is 60o crossflow type which has a very high specific surface

area of 125 m2/m3. In comparison, rock media filter has only 50 m2/m3. Crossflow filters tend

to have superior wetting characteristics and are able to nitrify better under low BOD loadings.

Table 1. BTF specifications (data supplied by CH2M Beca)

Components Dimensions

Diameter (centre column 1.2 m) 32.5 m

Depth 6 m

Area 828 m2

Total volume 4977 m3

Media (60o crossflow polypropylene) volume

Media total area (calculated from 125 m2/m3)

4971 m3

621,375 m2

Minimum flow to BTF to maintain required wetting 32400 m3/d

375 L/s

Wetting rate 39 m3/m2/d

BOD load

Original design

Current annual average

Current summer average

Current summer 95th percentile

Current winter average

Current winter 95th percentile

3780 m3/d (0.76 kg/m3/d)

3489 m3/d (0.70 kg/m3/d)

4096 m3/d (0.82 kg/m3/d)

6261 m3/d (1.26 kg/m3/d)

2889 m3/d (0.58 kg/m3/d)

4490 m3/d (0.90 kg/m3/d)

Aeration by 3 fan aerator with positive pressure 3470 L/s per fan





As shown Table 1, the minimum BTF flow rate to keep filters wet is 32,400 m3/d (375 L/s). If the

inflow drops below 375 L/s, a proportion of the treated BTF effluent is fed to the Feed Pump

Station where raw effluent has been screened and brought in to make up for 375 L/s. If the

inflow remains between 375 and 450 L/s, the BTF effluent is not recirculated. However, if it

exceeds 450 L/s, any excess inflow wastewater will be directed to the Outfall Pump Station.

The recirculating feature is also critical to the quality of the BTF effluent, with high recirculation

resulting in high quality effluent.

Data quality

Conventional wastewater engineering practices require BTF (biological trickling filter) designs

and operations be based mainly on BOD, COD and hydraulic loadings. For the above reason,

the performance of the BTF has often been measured on the basis of BTF’s ability to treat BOD

or COD.

As stated, modern BTFs with efficient plastic filter media and sophisticated aeration are not

only efficient in treating COD and BOD, but excellent in promoting N mineralisation and

assimilation (ammoniacal-N → organic-N) and nitrification-denitrification processes. Modern

BTFs are capable of treating N as a contaminant effectively. Unfortunately, the above feature

is not monitored or operated proactively.

In New Zealand, much of the wastewater treatment plant N treatment performance monitoring

is based on regional council compliance monitoring requirements. If the performance is

monitored outside consent conditions, it is monitored during the commissioning of the plant

or during any troubleshooting. The best way to monitor for BTF N treatment efficiency is to

monitor for influent TKN (which contains organic-N and ammoniacal-N) and, ammoniacal-N

and effluent TKN, ammoniacal-N and nitrate-N.

In the case of the GDC BTF, much of the effluent-N monitoring has been from NIWA’s

wastewater trial conducted between 2013 and 2016. Without such data, it is difficult to

recognise the existing BTF’s capability of treating N as one of the primary contaminants. Much

of the GDC BTF effluent-N output data had been collected between May 2013 and April 2014





and after a long pause another set of data were collected between November 2015 and August

2016. Given the high wetting rate of the BTF and the high potential for BTF effluent

recirculation, accurate estimates of contaminant loadings can only be possible with access to

actual daily BTF dosing volume data. Such data were not available at the time of preparing this

memo.

Raw wastewater Volume

As shown in Figure 1, the raw wastewater inflow ranged from 5000 to 40,000 m3/d between

2013 and 2016. During the period, trend analysis (dotted line in Figure 1) indicates the inflow

trend was around 12500 m3/d between May 2013 and August 2016. Given the BTF design

criteria require a minimum volume loading of 32,400 m3/d (or 375 L/s), it is highly likely except

for one occasion of high inflow, for the remainder of the period the BTF effluent was

recirculated to make up for the required BTF wetting rate dosing.

Figure 1. BTF wastewater inflow between 22 May 2013 and 10 August 2016

As stated, actual BTF daily dosing volume is critical in estimating mass loading of contaminants

to BTF, particularly BOD and COD. During the preparation of this memo, the actual daily BTF

dosing volume was not available. Therefore, the loading estimates based on raw daily effluent

volume in this memo have to be treated with caution. As for the contaminant level analyses,

0

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Raw effluent daily inflow (m3)





the dosing volume is not critical. For this reason, the data analyses on contaminant levels can

be considered as reasonably accurate.

Wastewater COD and BOD inputs between May 2013 and August 2016

As shown in Figure 2 by trend lines (dotted), there has been a steady increase in COD and BOD

loading since 2013 with COD increasing from 500 to 550 g/m3 and BOD increasing from 200 to

250 g/m3. It is noteworthy that between October 2014 and March 2015 the COD and BOD

loadings had more than doubled, >1000 and 500 g/m3 respectively. The reason for this is not

known. Judging by the wastewater volume input over this period, with the exception of

September and October 2014, the wastewater volume had been steady within the 12,500 m3/d

value.

Figure 2. Influent BOD and COD levels between May 2013 and August 2016

Figure 3 is an integration of daily raw wastewater influent volume, BOD and COD levels which

shows daily mass BOD and COD loadings to BTF. There has been a steady increasing trend in

BOD loading from 2 to 2.5 t/day and COD loading from 4.5 to 6.0 t/day. As expected, from the

high BOD and COD levels found over the October 2014 and March 2015 periods, a very high

daily loading of COD (>10 tonnes/day) and BOD (>4 tonnes/day) were also observed for the

above 6 month period.

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COD and BOD levels in the influent (g/m3)

Series1 Series2 Linear (Series2) Linear (Series1)





As for BTF performance in relation to BOD and COD treatment, Figure 4 illustrates that on

average the COD level in BTF effluent was around 250 g/m3 and BOD was around 90 g/m3

during early 2013. Despite the increasing COD and BOD loading trend, COD output was around

200 g/m3 and BOD was <50 g/m3 in late 2016. Considering the influent BOD and COD input

trend in 2016 being 250 and 550 g/m3 respectively (Figure 2), the treatment efficiency achieved

in treating BOD and COD was 80% and 64% respectively.

Figure 3. BOD and COD daily loading to BTF (in kgs/day)

Figure 4. BOD and COD levels in BTF Effluent between May 2013 and August

2016

02000400060008000

100001200014000160001800020000

Daily COD and BOD loading in kg


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BOD and COD BTF Effluent (g/m3)






The median COD and BOD calculated by NIWA over the 3.5 year period (between 2012 and

2015) was 220 and 75 g/m3 respectively (Table 5 in The assessment of sludge and effluent

characteristics from Gisborne biological trickling filter and recommendations for wetland

sludge treatment NIWA September 2015 report to GDC).

As stated, it must be noted that given the recirculation of BTF effluent with raw effluent on

occasions of inflow <375 L/s, ideally the above analyses have to be performed with the volume

actually discharged into the BTF. For the above reasons, the estimates for the BOD and COD

mass loadings have to be treated with caution.

Nitrogen species treatment by BTF

Typically, N species are measured differently in the influent and effluent. Given nitrification is

not expected within the influent, NO3 is not monitored in the influent. Urea-N from urine input

is also not monitored since urea in human urine would have hydrolysed to form ammoniacal-

N when reaching BTF. For the above reasons, only TKN (total kjeldhal-N which measures both

ammoniacal-N and organic-N together) and ammoniacal-N are measured in the influent.

Figure 5. TKN and ammoniacal-N levels in the BTF Influent between

February 2011 and April 2014

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There was limited dataset on ammoniacal-N and TKN on both BTF influent and effluent. As for

the influent, the data set available was limited between February 2011 and April 2014. Judging

by the above limited dataset, on average, TKN level reached around 40 g/m3 and ammoniacal-

N was around 26 g/m3 (Figure 5), which means the organic-N component was 14 g/m3. It is

noteworthy that TKN and ammoniacal-N were in greater levels over all summer periods

between 2011 and 2014, with the converse applying over the winter periods.

Within the limited ammoniacal-N and nitrate-N data available between May 2013 and August

2016, it was clear between May 2013 and April 2014 there was considerable nitrification

resulting in around 7.5 to 8.0 g/m3 nitrate-N and 3.0 to 4.0 g/m3 ammoniacal-N. This trend,

however, reversed in 2015/16 year with greater ammoniacal-N and lower nitrate-N outputs.

The reason for this was not clear.

Sometimes under heavy COD or BOD loadings, BTF could reduce the extent of nitrification.

However, judging by the concentration of COD and BOD, they were steady except for a short

period between September 2014 and March 2015 (Figure 2) there were considerable loadings.

Despite this sustained heavy loading over the short 6 month period, I consider the COD and

BOD treatment output of the BTF was being steady (Figure 4).

Figure 6. BTF effluent ammoniacal-N and nitrate-N between May 2013 and

August 2016

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Nitrate (blue) and Ammonium (red) output from BTF (g/m3)






Unless more details of BTF operation over such periods were known (e.g. extent and timing of

filter aeration and actual dosing volumes), it is difficult to identify the reasons for the reversing

BTF nitrification trend. What is very clear is that judging by the May 2013 to April 2014 dataset,

BTF was very capable of nitrifying ammoniacal-N. Whilst, the overall combined ammoniacal-N

and nitrate-N (dissolved inorganic-N) levels remained similar at around 10-11 g/m3 over both

nitrifying and non-nitrifying periods, from a tertiary treatment viewpoint, having greater

nitrate-N levels is considered as an advantage to further treat N by denitrification (e.g. using

constructed wetlands).

The BTF total-N treatment efficiency can be assessed by the difference between BTF influent

TKN (since no nitrate-N is found in the influent, TKN can be considered as total-N) and BTF

effluent nitrate-N + TKN.

From a total-N treatment efficiency viewpoint, judging by the TKN of 40 g/m3 over the May

2013 and April 2014 trend line (Figure 5) and the BTF effluent TKN (17 g/m3) and BTF effluent

nitrate-N (7 g/m3) derived from the respective trend lines (Figure 8), the combined total-N

output of 24 g/m3 indicates that the BTF treatment efficiency of total-N was around 40%, but

with TKN treatment alone was 58%.

Figure 7. BTF effluent TKN levels between March 2012 and April 2014

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Figure 8. TKN and nitrate-N levels in BTF effluent between May 2013 and

April 2014

The median TKN, ammoniacal-N and nitrate-N calculated by NIWA in the BTF effluent over the

3.5 year period (between 2012 and 2015) was 14.1, 0.8 and 5.6 g/m3 respectively (Table 5 in The

assessment of sludge and effluent characteristics from Gisborne biological trickling filter and

recommendations for wetland sludge treatment NIWA September 2015 report to GDC).

Phosphorus treatment

As for phosphorus treatment of the BTF, with the limited total phosphorus (TP) data set

available between May 2013 and April 2014, it was clear that BTF was not efficient in treating

P (Figure 9). The influent-P levels were often found to be greater than the BTF effluent-P levels.

This was expected given the high bacterial assimilation of N and P which resulted in high

sludge output from BTF (mean of 180 g/m3, Figure 10). The TP level in the BTF effluent was

considered as 6 g/m3 from the trend line in Figure 9.

The mean TP calculated by NIWA in the BTF effluent over the 3.5 year period (between 2012

and 2015) was 3.9 g/m3 (Table 5 in The assessment of sludge and effluent characteristics from

Gisborne biological trickling filter and recommendations for wetland sludge treatment NIWA

September 2015 report to GDC).

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TKN and Nitrate-N in BTF Effluent (g/m3)






Figure 9. Total-P levels in BTF influent and effluent between May 2013 and

April 2014

Contaminant assessment following solids removal by NIWA

Solid removal of the BTF effluent could result in substantial amount of TSS, BOD and TKN

removal. Considering the mean TSS estimated by NIWA over the 3.5 year period being 173

g/m3, the solid settling by NIWA had resulted in a TSS level of 6.4 g/m3 (Table 2) which was

considered as 96% solid removal.

Figure 10. Total suspended solid levels in BTF effluent between January

2011 and February 2016

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Similarly, a mean estimate of BOD of 92 g/m3 in the BTF effluent resulted in a BOD level of 7.9

g/m3 following clarification which was 91% BOD removal efficiency. TKN removal was also

good with clarification with 15.7 g/m3 BTF effluent resulting in 7 g/m3, which was 55% removal.

Greater bacterial assimilation of N will result in high organic-N (primarily dead bacterial cells)

which can be easily removed by solid removal rather than resorting to expensive N treatment

systems.

Table 2. BTF effluent quality following NIWA removal of solids

Date TSS BOD TKN NH4-N Org-N NO3-N Total-N TP DRP

23/2/2016 8.2 11.0 4.9 2.9 2.0 6.90 11.80 3.3 2.7

29/2/2016 12.0 11.0 6.6 4.0 2.6 0.12 6.72 4.2 3.6

15/3/2016 2.6 5.9 5.4 3.5 1.9 0.45 5.85 4 0.0

30/3/2016 5.6 6.8 3.5 2.1 1.4 2.60 6.10 3.5 2.9

12/4/2016 6.8 8.7 1.7 0.4 1.3 0.88 2.58 3.1 2.4

26/4/2016 7.8 8.4 2.6 0.9 1.6 4.40 7.00 2.9 2.3

10/5/2016 7.6 4.7 4.3 2.8 1.5 3.00 7.30 3.8 4.0

24/5/2016 4.2 7.7 13.0 10.0 3.0 1.70 14.70 3.3 3.6

7/6/2016 2.0 8.4 11.0 8.0 3.0 0.76 11.76 3.5 2.7

21/6/2016 7.4 6.1 7.7 5.8 1.9 1.50 8.20 3.7 3.3

5/7/2016 5.0 8.1 7.1 5.3 1.8 1.90 9.00 2.9 2.5

19/7/2016 4.0 7.8 9.3 7.1 2.2 2.20 11.50 3.6 3.4

2/8/2016 5.8 9.1 14.0 11.0 3.0 3.70 17.70 3.7 3.2

17/8/2016 8.5 7.9 7.7 6.0 1.7 2.00 9.70 2.8 2.6

30/8/2016 8.8 8.0 6.7 4.8 1.9 3.20 9.90 2.1 1.9

Mean 6.4 7.9 7.0 4.9 2.1 2.35 9.32 3.4 2.7

Modern BTFs are capable of promoting high N assimilation (conversion of ammonical-N into

organic-N or bacterial cells) which must also result in high P assimilation. However, NIWA’s

sludge removal did not appear to result in high P removal with influent TP mean was being 3.9

g/m3 and after sludge removal the TP level remained at 3.4 g/m3 (Table 2).

Having an additional BTF in parallel can promote BTFs to produce high quality effluent. The

current BTF has demonstrated that it provides easily settleable solids which is a good

characteristic that must be maintained when operating two parallel BTFs.





As seen in Table 3, it has been predicted that with double BTFs, considerable reduction in

ammoniacal-N and TKN can be achieved by nitrification-denitrification and N assimilation.

However, it is noteworthy the TP performance remains status quo (3.8 g/m3) despite engaging

double BTF technology.

Table 3. Predicted BTF effluent quality following clarification (adopted from

NIWA GDC wetland report)

Single BTF Double BTFs

Summer Winter Summer Winter

Temp. (°C) 22 17 15 22 17 15

BOD5 (mg/L) 6 8 9 3 4 4

TSS (mg/L) 30 30 30 30 30 30

NH4-N

(mg/L)

0.8 5 10 0.1 0.2 0.2

NOx-N

(mg/L)

0.9 0.8 0.4 2.3 2.5 2.5

TKN (mg/L) 3.3 7.4 12.7 2.6 2.8 2.8

TN (mg/L) 4.2 8.2 13 4.9 5.3 5.5

TP (mg/L) - - - 3.5 3.8 3.7

Conclusions

1. Data analyses were not straight forward given the ad hoc way in which the data had

been collected.

2. Based on the data assessed between 2013 and 2016, the current BTF’s effluent output

is considered as follows (g/m3):

COD = 200

BOD = 50

TSS = 180

TKN = 17

Ammoniacal-N = 8

Organic-N = 9

Nitrate-N = 4

TP = 6 (based on 2013-2014 data)





3. The current BTF treatment performance can be further optimised to improve N

treatment efficiency, mainly by N assimilation (conversion of ammoniacal-N to sludge-

N) and by promoting conditions conducive to nitrification. This would require data

collection on key contaminants such as SS, BOD, TKN, ammononiacal-N and nitrate-N

and aeration, raw influent volume, recirculation of BTF effluent, BTF or effluent

temperature and influent and effluent pH at least during the optimisation period.

4. The above step is critical in the decision making process in designing and implementing

the additional parallel BTF to improve the overall N output to be further treated by

methods such as constructed wetlands or for maintaining single BTF with constructed

wetland.


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