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10/13/2014
1
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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93
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
94
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.