OPTIONS FOR DECENTRALIZED AND
CENTRALIZEDWASTEWATER PLANTS
WATER RECLAMATION DIVISION
NATIONAL WATER SUPPLY & DRAINAGE BOARD
06TH AUGUST 20241

Safe Sanitation Service Chain

User Interface

Containment

Conveyance

Treatment

Reuse/Safe Disposal

Types of Sanitation Solutions
1. Onsite Sanitation
Systems Provide of safe containment/ treatment for wastewater generated at scatteredhousing units, institutions, etc.
 Septic Tanks + Soakage Pits/ associated disposal systems

2. Faecal Sludge
Management Systems

Facilitate safe disposal of faecal sludge from septic tanks /DEWATS to meet safe
sanitation requirements

3. Decentralized
Wastewater Treatment
Systems (DEWATS)

 Suitable for locations where significant amount of wastewater generation
 Limited access to City Sewerage Systems due to distant location, costimplications.
 Suitable for clustered houses located away from existing city sewerage schemes.
E.g. Condominium Housing, Estate Sector Housing, Schools, Hospitals, Prisons,Institutions, Public toilets in townships/pilgrim places, etc.

4. Centralized
Wastewater Collection& Treatment Systems

 Economical for highly urbanized compact areas where limited space for onsite
WW treatment.
 Significantly large amount of wastewater generation.
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Important Facts on DEWATS
Usually function without outside energy or low energy inputs.
Tolerant towards inflow fluctuations
Usually does not require sophisticated technical equipment
If provided, it ensures more reliable operation and good stable effluent quality with energy input.
Based on a modular, technical configuration concept.

Does not require higher operation andmaintenance skills.

Appropriate combinations of treatment modules can beselected, depending on the required treatment
efficiency, costs, land availability, etc.

Most operational & maintenance tasks can becarried out by the users, some maintenanceservices might require a local service provider.
In some cases, both operation and maintenancecan be delivered by a service provider
Consider the socio-economic
environment of a given location.

DEWATSCan reduce pollution load to meet legal requirements.
Generated solid waste (sludge) must be handled,
treated and disposed of in accordance with hygiene andenvironmental standards

Neglecting these conditions will
result in the failure of the technology
Can be constructed form locally available materials.
Can be implemented by the local workforce provided withhigh quality standards in planning.

Treatment Process for DEWATS• Typically use of the physical and biological (anaerobic, aerobic or combination) treatment processes toreduce and remove pollutants from wastewater.

Sedimentation

AnaerobicDigestion• removal of easily settleable solids• anaerobic fermentation of settled solids• removal of sludge• removal of easily degradable organic solids• mineralization of suspended or dissolved organic compounds, biogas production• settling of mineralized particles, collection and ventilation of biogas• removal of sludge

AerobicDecomposition• removal of easily and more difficult degradable solids• mineralization of suspended or dissolved organic compounds• settling of mineralized particles• removal of sludge

Post-treatment• removal of suspended digested solids, active bacteria mass and pathogens• settling of finest suspended solids, removal of algae• retaining of living and dead algae• removal of sludge

5Treatment Options for DEWATS
Sedimentation

AnaerobicDigestion• Septic Tank / Holding Tank
• Sedimentation Tank
• Anaerobic Baffled Reactor• Anaerobic Filter (attached growth system)• Anaerobic Pond
• Fully mixed digesters or Imhoff tanks• UASB (Upflow Anaerobic Sludge Blanket) Reactor/ Anaerobic Digester

Aerobic
Treatment /Decomposition• Facultative/Oxidation Ponds• Aeration Tank/Moving Bed Biofilm Reactor (MBBR) - aeration + attached growth• Trickling Filters• Down Hanging Sponge Reactors (attached growth system)

Post-treatment• Polishing Ponds/Maturation Ponds• Gravel Filters/Constructed Wetlands• Floating Wetlands• Disinfection

Packaged Treatment Units Combination of above treatment steps
Ex.:- Johkasou system, Fixed Bed Biofilm Reactor (FBBR), BioKube Treatment Systems

DEWATS Technology Selection
Appropriate technical configuration depends on the: volume of wastewater quality of wastewater local temperature underground conditions
 land availability
 costs
 legal effluent requirements (CEA Effluent Discharge Standards)
 cultural acceptance and social conditions
 final handling of the effluent (safe discharge or reuse)Examples for De-centralized Wastewater Treatment Systems
 SLS 745 Part-I and Part-II design guidelines for designing Septic Tank, Anaerobic Filter, Soakage Pit and otherassociated onsite wastewater disposal systems
Septic Tank + Soakage Pit (In-situ /Precast Construction)Suitable for low seasonal ground water table (> 2.5 m) with high soil percolation capacity
Able to remove about 50% –60% of pollutant load in the sewage
Well designed soakage pit further removes pollutant and faecal contamination load when pass through soil
Hardly any O&M input requirement except annual desludging of septic tank

Limitations; Soil/site limitations (soil percolation
capacity and high ground water table)
 Availability of space At least 50 feet away from dug wells andsurface water sources
 Not suitable for reuseSeptic Tank + Seepage Trenches/Beds
 Suitable for high seasonal ground water table (>1.0 m) with moderate/high soil percolation capacity
 Hardly any O&M input requirement except annual desludging of septic tank

Limitations; Soil/site limitations (soil percolation
capacity and high ground water table) More land required compared to septictank + soakage pit system
 Landscape disturbanceSeptic Tank + Anaerobic Filter + Soakage Pits
 Suitable for individual dwelling or small commercial/institutional establishments
 Able to remove some pollutant loads below CEA discharge standards
 Hardly any O&M input requirement except annual desludging of septic tank
Successful implemented 3554 of similar systems under GPOBA project
Capacity – 1 m3/day
Treatment efficiency >90%, Effluent BOD5 < 30 mg/l

Septic Tank + Anaerobic Filter + Wetlands

Suitable for individual dwelling or small commercial/institutional establishments where more/sufficient land is available
Able to remove pollutant loads below CEA discharge standards
Possibility for effluent reuse preferably after disinfection
Hardly any O&M input requirement except annual desludging of septic tank and trimming and removal of excess wetland
plants and dried leaves.

Example of successful implementation of Septic tank + Anaerobic
biofilter + Wetland and Chlorination system for a house in Panadura.
Capacity – 1 m3/day
Treatment efficiency >90%, Effluent BOD5 < 20 mg/l

Septic tank + Up flow Anaerobic Filter + Wetlands + Reuse system
Jaffna University Female Hostel in operation since 2014
Capacity – 45 m3/day
Treatment efficiency >90%, Effluent BOD5 < 30 mg/l

Septic tank + Up flow Anaerobic Filter + Sub-surface Wetland and Chlorination system
Ratmalana Tsunami Resettlement Housing Scheme (324 Housing Units – 4 Storied buildings in 2011)
Capacity – 200 m3/day
Treatment efficiency > 90%, Effluent BOD5 < 30 mg/l

Septic tank + Up flow Anaerobic Filter + Constructed Wetlands
DEWATS at Adams Peak in operation since year 2014
Capacity – 65 m3/day
Treatment efficiency >90%, Effluent BOD5 < 30 mg/l

Packaged Type Treatment Units Sedimentation, Anaerobic Filter, Aeration Tank /MBBR, Storage and Disinfection
 Capacity usually 1-50 m3/day
 Needs energy input for aeration
 Good effluent quality & possible for reuse Requires periodic operation and maintenance Requires sludge disposal in every 6 months

Desludging of the Johkasou system at the Ministryof Water Supply

Other Types of DEWATS

Kegalle Hospital
Imhoff tank + Trickling Filter System

Defense Headquarters - Akuregoda
Activated Sludge Extended AerationTreatment System + Maturation Ponds
1200 m3/day•

High energy intensive treatment process

Peliyagoda Fish Market
UASB+ MBR System•••High energy intensive treatment process
Very good effluent quality
Reuse for floor washing

Centralized Wastewater Treatment Systems/Urban Sewerage Systems••••

Centralized large-scale plants that serve expansive municipal or regional service areas
Used high-tech equipment and high energy intensive process
Coupled with large sewer reticulation system
Need skilled work force for O & M
Kandy City WWMP(Oxidation Ditch)
Capacity – 14,000 m3/day

Ja Ela Ekala WWMP - (A2O)
Capacity – 7,500 m3/day

Population Coverage–55,000 (Domestic)

Population Coverage–8,000(Domestic) + Industrial/73,500 PE

Greater Kurunagala WMMP(AO)

Kataragama WMMP(Aerated Lagoons)

Capacity – 4,500 m3/day

Capacity -3,000 m3/day

Population Coverage–
25,000(Domestic)

Population Coverage– 12,000(Domestic)Thank You18

What is a Faecal Sludge
Treatment Plant (FSTP)?

• After Faecal Sludge/Septage is desludged/removed from Septic Tanks/Pit
Latrines, it has to be transported bymeans of Gulley Bowsers to Faecal
Sludge Treatment Plants(FSTPs) fortreatment.

• The treated wastewater has to becomplied with CEA discharge standards.
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Existing Faecal Sludge Treatment Plants in Sri Lanka
Mechanized Faecal Sludge Treatment Plants(FSTPs)- Activated Sludge

Ruwanwella Faecal Sludge
Treatment Plant (25m3/Day)

Weligepola Faecal Sludge Treatment
Plant (25m3/Day)

Thalawakele Faecal Sludge Treatment
Plant (25m3/Day)

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Existing Faecal Sludge Treatment Plants
Waste Stabilization Pond Systems
Mulathivu Faecal Sludge
Treatment Plant (25m3/day)

Kilinochchi Faecal Sludge
Treatment Plant (25m3/day)

Chilaw Faecal Sludge Treatment Plant(25m3/day)21