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Ponds and Pits (and more):Sanitation technologies in developing country settings

Matthew E. VerbylaENV 6510 Sustainable Development Engineering

PHC 6301 Water Pollution and TreatmentOctober 13, 2014

Agenda

1. Latrines

– Pit/VIP

– Pour-flush

– Composting

2. Septic Systems

– Septic tanks

– Leach pits

– Leach fields

3. Bioreactors

– Imhoff tanks

– UASB reactors

– Trickling Filters

4. Stabilization Ponds

– Anaerobic

– Facultative

– Maturation

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1 billion people.

0.7 billion people.

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0.7 billion people.

Conventional Pit Latrine

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Calculate air exchange rate(HVI recommends 8 h-1):

𝑉𝑒𝑛𝑡𝑖𝑙𝑎𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑒 (𝑚3/ℎ)

𝑉𝑜𝑙𝑢𝑚𝑒 𝑆𝑢𝑝𝑒𝑟𝑠𝑡𝑟𝑢𝑐𝑡𝑢𝑟𝑒 (𝑚3)

Ventilated Improved Pit (VIP) Latrine

Orient the vent pipe, slab, and shelter with respect to

the prevailing wind direction

Pour-Flush Latrines

• Pit can be directly over or offset from the toilet

• Water seal trap

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Composting Latrine

Composting Latrine

• Also called eco-san latrine

• Good for areas with high water table

• Desiccant added after each use (e.g. saw dust, ash)

• Typically double vault with urine diversion

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Many ways to separate urine

Photo from Danny Hurtado

Photo from Eric Tawney

Photo from Beth Myre

Photo from Ryan Shaw

Photo from Ryan Shaw

Transfer of technologies between geographic regions

• First designed by Henry Moule (English priest)

• Became popular in the 1970s – 1980s in many of the Nordic countries (e.g. MullToa in Sweden)

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Source: WHO

Distribution of soil-transmitted helminthiases

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Sizing Latrines

• Accumulation rate is typically 0.02 – 0.09 m3/capita/year, depending on:

– Pit's proximity to water table

– Diet of users

– Types of anal cleansing materials used

• Old pit should be emptied or new pit should be dug when old pit is 80% full

Challenges and Opportunities with Operation and Maintenance

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LATRINE STRUCTURES

Out in the Open(not recommended)

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Thatch

Mud or Adobe

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Wood

Metal

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Brick

Concrete Block and Ceramic Tile

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Inside the Home

The “pit” is the most important part of toilet sustainability

New toilet #1

New toilet #2New toilet #4

New toilet #3

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Pathogen Destruction

Mehl et al. (2011)

pH vs. Helminth Survival

> 119-10

< 9

Moe & Izurieta (2003)

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Temperature vs. Helminth Survival

> 36°C

33 – 35°C

< 33°C

Moe & Izurieta (2003)

Solar Composting Toilet

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QUESTIONS ABOUT LATRINES?

1.5 billion people.

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Septic Systems

Design Considerations

• Tank configuration

• Structural integrity

• Water tightness

• Tank size

• Operation and maintenance

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Tank Configuration• Typically rectangular in shape, may also have interior baffle to

divide tank into two volumes (historically-based):

– 2/3 volume for sediments, 1/3 volume for scum

• Single-volume tanks have equal or better performance than two-compartment tanks

• Longitudinal baffles are recommended by Crites & Tchobanoglous(1998) because of improved performance and structural integrity

Single Compartment Tank

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Structural Integrity• Typically concrete, fiberglass,

or plastic

• Steel or wood used in past (not recommended)

• If concrete, walls and bottom of tank should be poured monolithically; top should be cast-in-place with rebar from walls extending into top slab

• Longitudinal baffles not only improve performance, also the structural integrity

Water Tightness1. Prior to installing tank,

fill with water and let sit for 24 hours

2. Some water will be absorbed by concrete, so refill and let site for another 24 hours

3. If no water leaks during first 24 hours, and <1 gallon water loss during second 24 hours, then the tank is acceptable

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Septic Tanks

• Sizing the tank (Based on empirical relationships)

– 1 – 2 bedrooms: 1,000 gallons

– 3 bedrooms: 1,500 gallons

– 4 bedrooms: 2,000 gallons

– > 4 bedrooms: ~5x average flow

DesludgingInterval (years)

Volume of Tank

(gallons)

3 2.8 · Qavg × PF

4 3.2 · Qavg × PF

5 3.7 · Qavg × PF

6 4.0 · Qavg × PF

Septic Tank Maintenance

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Design of Leach Systems

Terminology:Septic Tank vs. Leach System

“Pozo Séptico”“Fosa Séptica”“Tanque Séptico”“Pozo Ciego”“Pozo Negro”

Septic System with Septic Tank, Distribution Box, and Leach Field

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Leach/Seepage Pit (Cesspool/Cesspit)

Leach/Seepage Pit (Cesspool/Cesspit)

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Leach/Seepage vs. Cess

Leach Trench

Distribution pipe with

rock cover

1 – 3 ftrock

backfill

Vadose Zone

Perched Zone

Groundwater

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Leach Field

Leach Field

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Gravel-less Pipe

Chamber System

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Chamber System

Mound Systems

• Typically used for sites with high water tables or where soils have inadequate permeability

• Analogous to a raised leach field

• Uniform, large-grain sand (helps with evapotranspiration through capillary action)

• May require pump or siphon depending on site topography

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Leach System Design Considerations

1. Preliminary Site Evaluation

2. Local Regulations

3. Detailed Site Assessment

– Determine hydraulic capacity

– Locate wells, buildings, trees, etc.

QUESTIONS ABOUT SEPTIC SYSTEMS?

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Bioreactors

Pre-Treatment

• Very necessary in order to remove large, heavy, non-organic solids that can clog downstream components of the treatment system

• Must design for peak flow (often during rain event)

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Bar Screens

Grit Chamber

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Imhoff Tanks

• Developed in Germany in 1905

• Slightly improved version of septic tank

• Includes separate solids digestion compartment; wastewater does not flow through there

Imhoff Tanks

• Became very popular in developing countries as treatment units for neighborhoods, small cities, and towns

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Comparison

SEPTIC TANKS IMHOFF TANKS

Venti lación

Zona de

sedimentación

zona de digestión

lodoSludge

Biogas

Digestion Zone

SedimentationZone

Effluent

Influent

Ventilation

Ventilación

decantación

bolas de

biogas

lodo

retenedor de

espuma

efluente

afluente

Sludge

Biogas

Effluent

Influent

Ventilation

Decant

Scum

As with any technology…

If it doesn’t get properly maintained…

…it won’t work!

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Upflow Anaerobic

Sludge Blanket

(UASB) Reactor

UASB Reactor Design Criteria• Volumetric Hydraulic Load and Retention Time

𝜆𝑣 =𝑄

𝑉θ =

𝑉

𝑄

V – Reactor volume (m3); Q – Mean flow rate (m3 d-1)

λv – Volumetric hydraulic loading rate (m3 m-3 d-1)

θ – Hydraulic retention time (days)

• Hydraulic loading rate should not exceed 5.0 m3 m-3 d-1

• Retention time no less than 4 hrs. at peak flow

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UASB Reactor Design Criteria• Organic Load equation

𝜆𝑜 =𝑄𝐶𝑜𝑉

• Originally developed for industrial wastewater treatment with organic loading rates of 10 – 15 kgCOD/m3*d

• Domestic wastewater has ~2.5 to 3.5 kgCOD/m3*d, but technology still works in warm climates

• Reactors for domestic WW should be designed based on hydraulic loading rate, with average upflow velocity of 0.5 to 0.7 m/hr (peak flow velocities of 1.5 – 2.0 m/hr can be tolerated for up to 2 – 4 hrs.

v = Q / A

UASB reactors followed by rock filters in periurban Cochabamba, Bolivia

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• Sized based on surface area of medium, flow, volume, and temperature constant

• Fly control is important

• Good for steep slopes

Trickling Filter

Source: Mara (2003)

Media with Higher Surface Area

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Trickling Filter

Locally-Available

Media

• Gravel

• Plastics

• Peat

• Rubber

• Recycled materials

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Stabilization Pond Systems (Lagoons)

Typical Configurations

Effluent

Polishing

Pond EffluentInfluent

(Raw Sewage)Bioreactor

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Anaerobic Ponds

Anaerobic Ponds

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Design Considerations• Typically 3 – 5 m deep, theoretically sized by volumetric BOD loading:

𝑉 =𝐶𝑄

𝜆𝑣

V – Pond volume (m3)

C – Mean influent concentration of BOD5 (g BOD5 m-3)

Q – Mean influent flow rate (m3 d-1)

λv – Volumetric loading rate (g BOD5 m-3 d-1)

• Mara’s (2003) guidelines: choose loading rate based on temperature during the coldest month of the year

Temperature(°C)

Volumetric Loading Rate(g BOD5 m-3 d-1)

Anticipated BOD removal (%)

<10° 100 40

10 to 20° 20 × T – 100 2 × T + 20

20 to 25° 10 × T + 100 2 × T + 20

>25° 350 70

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BOD5 Removal vs. Volumetric Loading for Anaerobic Ponds in Sao Paulo, Brazil

Control of Inhibitory Substances

• May inhibit the growth of anaerobic microbial community

Substance Moderate Inhibition Strong Inhibition

Sodium 3,500-5,500 8,000

Potassium 2,500-4,500 12,000

Calcium 2,500-4,500 8,000

Magnesium 1,000-1,500 3,000

Sulfides 200 >200

Source: EPA (2011), originally from Parkin & Owen (1986)

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High-Rate Anaerobic Pond

Potential for Biogas Recovery

Municipal Wastewater Treatment Pond SystemSanta Cruz, Bolivia

Covered Anaerobic Pond in China (Photo credit: Heinz-Peter Mang)

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Facultative Ponds

Facultative Pond Design• Typically 1.5 – 2 m deep, sized by surface BOD5 loading rate:

𝐴 =10𝐶𝑖𝑄

𝜆𝑠

A – Pond surface area (m2)

Ci – Mean influent concentration of BOD5 (g BOD5 m-3)

Q – Mean influent flow rate (m3 d-1)

λs – Surface loading rate (kg BOD5 ha-1 d-1)

• The surface loading rate can be chosen using two methods:

1. Based on the temperature during the coldest month (Mara 2003)

𝜆𝑠 = 350 1.107 − 0.002𝑇 𝑇−25 (units: kg ha-1 d-1)

𝜃 =𝐴𝐷

𝑄(recommending HRT (𝜃) of at least 5 days)

2. Based on the theoretical rate of O2 production by algae (Oakley 2005)

106𝐶𝑂2 + 65𝐻2𝑂+ 16𝑁𝐻3 + 𝐻3𝑃𝑂4solar radiation

𝐶106𝐻181𝑂45𝑁16𝑃 + 118𝑂2

(𝐶106𝐻181𝑂45𝑁16𝑃 represents algal biomass)

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106𝐶𝑂2 + 65𝐻2𝑂 + 16𝑁𝐻3 + 𝐻3𝑃𝑂4solar radiation

𝐶106𝐻181𝑂45𝑁16𝑃 + 118𝑂2

• 1.55 kg of O2 is produced for each kg of algal biomass

• Algae can theoretically produce 1 kg of biomass with 24,000 kJ of sunlight, but the efficiency of this energy conversion is only ~2 – 7% (Oakley 2005)

𝜆𝑠 =𝐼𝑠 ⋅ 𝑒 ⋅ 1.55

kg O2kg algal biomass

24,000kJ

kg algal biomass

λs – Maximum surface loading rate (g BOD5 m-2 d-1)

Is – Solar insolation (kJ m-3 d-1)

e – Efficiency of energy conversion by algae (%)

• 1.55 kg BODu ≈ 1 kg BOD5

Facultative Pond Design (continued)

Facultative Pond Design (continued)

• BOD removal

𝐶𝑒(𝑢𝑛𝑓𝑖𝑙𝑡𝑒𝑟𝑒𝑑) =𝐶𝑜

1 + 𝑘1𝜃𝑓

𝑘1(𝑇) = 𝑘1(20) 1.05𝑇−20

𝑘1(20) = 0.3 𝑑𝑎𝑦𝑠−1 for primary facultative ponds0.1 𝑑𝑎𝑦𝑠−1 for secondary facultative ponds

𝐶𝑒 𝑓𝑖𝑙𝑡𝑒𝑟𝑒𝑑 = 𝑓𝑛𝑎 𝐶𝑒(𝑢𝑛𝑓𝑖𝑙𝑡𝑒𝑟𝑒𝑑)

𝑓𝑛𝑎 = 0.3 (for design purposes)

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Nitrogen Pathways in Ponds

Design for Ammonia RemovalMethod specified by Mara (2003) for Removal of NH4

+ (as N)from Pano & Middlebrooks (1982) and Silva et al. (1995)

• Temperatures < 20°C:

𝐶𝑒 =𝐶𝑜

1 +𝐴𝑄∙ 0.0038 + 0.000134 ∙ 𝑇 ∙ 𝑒 1.041+0.044∙𝑇 𝑝𝐻−6.6

• Temperatures 20-25°C:

𝐶𝑒 =𝐶𝑜

1 + 5.035 × 10−3 ∙𝐴𝑄

𝑒 1.54∙ 𝑝𝐻−6.6

• Temperatures > 25°C:

𝐶𝑒 =𝐶𝑜

1 + 8.65 × 10−3 ∙𝐴𝑄∙ 𝑒 1.727∙ 𝑝𝐻−6.6

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Maturation Ponds• Typically ~1 m deep, sized based on the removal of pathogens

(bacteria, viruses, protozoa, helminths) or nutrients (N & P)

• Design for helminth egg removal (Ayres et al. 1992):

𝐶𝑒 = 𝐶𝑜 0.41𝑒−0.49𝜏+0.0085𝜏2

• Design for coliform removal (von Sperling 1999, 2002, 2003):

𝐶𝑒 = 𝐶𝑜4𝑎

1 + 𝑎 2𝑒

1−𝑎2𝛿

𝑎 = 1 + 4𝑘𝐵 𝑇 𝜏𝛿 𝑘𝐵 𝑇 = 0.92𝐷−0.88𝜏−0.331.07𝑇−20

τ – Theoretical hydraulic retention time (volume/flow)

δ – Dispersion number (approximated as (length/width)-1, i.e. δ = (L/B)-1)

kB(T) – Coliform pseudo-first-order die-off coefficient (d-1)

Ce – Effluent concentration, Co – Influent concentration

L – pond length, B – pond breadth, D – pond depth

Coliform Removal in Ponds

• The coliform die-off coefficient (kB) can be estimated based on the depth of the pond (D), the temperature (T) and the hydraulic retention time (τ):

𝑘𝐵 𝑇 = 0.92𝐷−0.88𝜏−0.331.07𝑇−20

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Helminth Egg Removal in PondsAyres et al. (1992)

Limitations (or opportunities?) for Pond Systems

• High nutrient levels in effluent

• High BOD and suspended solids (algae) in effluent

• Requires large areas of flat land

• Attracts all kinds of animals (bad?)

• Odor issues

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As with any technology…… if it doesn’t get maintained, it won’t work

QUESTIONS ABOUT POND SYSTEMS?

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Resource Recovery Priorities for Small Cities and Towns

RECOVER ENERGY? RECOVER WATER & NUTRIENTS?

Can/should small cities/towns do both?

Resource Recovery Priorities for Small Cities and Towns

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Photo credit: Pablo Cornejo

Pond Systems Anaerobic Reactors

SAFE WATER REUSE

Protozoa Helminth Eggs Viruses Bacteria

Requires post-treatment

HARVEST

BIOGAS!

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September 2014

The town of Cliza, located in the water-scarce upper

Cochabamba Valley of Bolivia, inaugurates their new

wastewater treatment system. Their former system, which

used stabilization ponds, has been decommissioned.