Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Up-scaling Basic Sanitation for the Urban Poor (UBSUP)
Overview of Secondary Treatment Facilities Requirements
and Siting of On-site Systems
Prepared by: UBSUP Technical Working Group
Date (16-02-2013)
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Table of Content
EXECUTIVE SUMMARY
4
1.1.1
What Is a Septic Tank?
1.1.2
Septic Tanks siting Considerations
1.2.0 SOAKAWAY
1.2.1 What Is a Soak away?
1.2.2 Soak-away siting considerations
1.3.0 CONSERVANCY TANK
1.3.1 What is a conservancy tank?
1.3.2 Conservancy tank siting considerations
1.4.0 Unplanted Drying Beds
1.4.1 What Is an Unplanted Drying Bed?
1.4.2 Unplanted Drying bed siting considerations
1.5.0 Co-composting Facility
1.5.1 What is co-composting?
1.5.2 Co-composting facility siting considerations
1.6.0 DEWATS
1.6.1 What Are DEWATS?
1.6.2 DEWATS siting considerations
1.7.0 SUMMARY OF CONSIDERATIONS IN SITING DIFFERENT SANITATION FACILITIES
1.8.0. References
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Acronyms and Abbreviations
BoQ
Bill of Quantities
DEWATS
Decentralised Waste Water Treatment
EPP
Ecosan Promotion Project
FBO
Faith Based Organizations
GIZ
Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH
IO
International Organizations
KARI
Kenya Agricultural Research Institute
KeBS
Kenya Bureau of Standards
MDGs
Millennium Development Goals
MoA
Ministry of Agriculture
MoF
Ministry of Finance
MPHS
Ministry of Public Health and Sanitation
MWI
Ministry of Water and Irrigation
NEMA
National Environmental Management Authority
NGO
Nongovernmental organisation
O&M
Operation & Maintenance
UBSUP
Up-scaling of Basic Sanitation for the Urban Poor
UDDT
Urine diversion dehydrating toilet
UDT
Urine diverting dry toilet
WASREB
Water Services Regulatory Board
WHO
World Health Organisation
WRMA
Water Resource Management Authority
WSP
Water Services Providers
WSTF
Water Services Trust Fund
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
EXECUTIVE SUMMARY
This document describes the different technical options for secondary treatment with the specific
objective of providing guidelines and criteria for siting the facilities.
The secondary facilities that have been considered in this document are the facilities that have been
identified to have potential of adoption by UBSUP.
These facilities are: Septic tanks
 Soak away pits
 Conservancy tanks
 Co- composting beds
 DEWATS
Proper siting of these facilities ensures protection against water sources contamination i.e. wells and
streams.
The safety and effectiveness of a well and streams depends greatly on its location in relation to
sanitation facilities.
It is important to maintain safe distances between private ground water wells and possible sources
of contamination.
The Possible sources of contamination and minimum distances from wells include:

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Septic Tanks and conservancy tanks, 50 feet from well
Livestock yards, Silos, Septic Leach Fields (including soak away pits) , 50 feet from well
Petroleum Tanks, Liquid-Tight Manure Storage and Fertilizer Storage and Handling (including
drying and composting beds), 100 feet from well
Manure Stacks, 250 feet from well
Source:EPA
Ground water can become unusable if it becomes polluted and is no longer safe to drink.
Contaminated ground water can be the result of:



Microbial contamination (fecal contamination from feedlots)
High concentrations of naturally-occurring contaminants, such as arsenic and radon (depends
highly on the geology of the land surrounding the well)
Local land use practices (fertilizers and pesticides)
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems

Problems with the integrity of nearby on-site septic systems
The diagram below gives a schematic illustration of the siting distances
Source: Southern Idaho Public Health; 2012; www.siphidaho.org
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
1.1.0 SEPTIC TANKS
1.1.1
What Is a Septic Tank?
A septic tank is basically a vessel buried underground, the purpose of which is the
collection, storage, and, to some limited extent, treatment of sewage. On a simple hierarchy
of sewerage facilities, the septic tank falls between the 'long-drop pit latrine', the function of
which is to store and bury human waste, and a piped connection to an fully functioning
sewage treatment works, the function of which is to convert human waste products into
mostly harmless end products.
A typical septic tank system normally operates by gravity, and consists of a tank and a
soakaway drain. Untreated wastewater from a property flows into the septic tank, where the
solids separate from the liquids. Some solids, such as soap scum or fat, will float to the top of
the tank to form a scum layer. Heavier solids, such as human and kitchen wastes, settle to the
bottom of the tank as sludge. Self forming bacteria in the tank help the system "digest" these
solids or sludge where a natural process of anaerobic decomposition occurs in the tank which
reduces the amount of solid matter and provides some treatment of the waste. The remaining
liquids flow out of the tank to a percolate into the soil (soakaway) and eventually taken up
through the root system of plants or added to the groundwater. Baffles built into the tank hold
back the floating scum from moving past the outlet of the tank.
But what you may not know is that an improperly sited, designed, installed or operated septic
tank system can pollute drinking and surface water, and cause many problems, especially
environmental. Because septic tank systems are underground, they are often ignored by
people who own or use them. However because septic tank systems are generally out of sight
they should not be out of mind. The effluent from a septic tank still contains about 70% of the
polluted matter in the sewage, and hence there is a need for further treatment of the liquid
from the tank.
Septic tank care is crucial to maintain a healthy septic system. The septic tank is a passive
system. There is nothing electrical or mechanical involved. Although the septic system is
actually quite self-sufficient, there are things you can to help the system work efficiently.
Microbes in your septic system will naturally break down the organic material that drains into
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
your septic tank. The broken down material and water will naturally drain out of the septic tank
and into the ground underneath it.
Some solids cannot drain out of the septic tank. This is normal. Regular septic
tank care requires you to pump out these solids every three to five years. The
size of your septic tank, the amount of use, and the kind of products you stick
in your drains will determine how often your septic tank will need to be pumped.
1.1.2
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Septic Tanks siting Considerations
Septic tank 50 feet from surface water
Septic tank 50 feet from all well
Septic tank 5 feet from foundation
Septic tank and drain field to be 6 feet apart
Avoid low lying areas
Avoid areas where root may grow towards the septic tank
1.2.0 SOAKAWAY
1.2.1 What Is a Soak away?
The soak away or percolation trench is an underground soil treatment system, which receives
partially treated sewage from the septic tank. The soil on a site must be suitable for a soak away to
work properly. It is noted here that the effluent from a septic tank is by no means fit (in terms of
health) for discharge into a water course (e.g., a river, vlei1 or an aquifer) or onto the ground where
it could be accessible to animals, humans included.
1 The word vlei is used predominantly in South Africa. It is an Afrikaans word derived from Dutch "vallei". In Afrikaans,
however, its meaning changed into "shallow minor lake", mostly of a seasonal or intermittent nature. Wikipedia
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Not all sites are suitable for septic tank and soak-away systems. Of primary concern is the type and
porosity of the soil at the site. Soils that are too coarse or too fine can limit the effectiveness of the
treatment system. Also the depth of the seasonally high water table or bedrock can also cause
problems. Some of these problems may possibly be overcome by altering the design of the septic
system. It is good practice to carry out percolation tests on the ground soil to ascertain its suitability
for a soak-away and generally you cannot improve an unsuitable site to the point where a soil
treatment system will work
In areas with high water table within a 600mm of the ground surface a soak-away may be typically
installed close to the surface of the ground, but care must be exercised as it is possible that the
polluted water may break through the ground surface. In other areas there may be clay ground
present close to the ground level, and where this occurs then it is very unlikely that the use of a
soak-away will prove satisfactory.
The size of your septic tank soak-away is determined by the size of not only your septic tank but the
size of your dwelling as well. You cannot simply construct a soak-away to the specifications that you
wish. There are other considerations including where the soaka-way is built. It cannot be built near
any open water such as a river or a vlei because of the possibility of seepage from the soil into the
water. This would result in contamination of the water source which could lead to many problems
including health concerns.
1.2.2 Soak-away siting considerations
 Soak-away pits to be sited 50 feet from all temporary surface water (ditches and canals)
 Soak-away to be 100,200, 0r 300 feet from surface water (stream, lakes, or rivers) depending
on soil type
 They should be 100 feet from all wells
 Soak-away to 10 feet from foundations and 20 feet from basements
 They should be at least 5 feet from property lines
 Areas where vehicles may compact such as drive ways, parking and roads should be avoided
 Steep slopes should be avoided; slopes less than 20% are preferred.
 Avoid areas where tree may grow towards and impact on the soak-away
 Avoid low lying areas where there are runoffs or storm water drains or where water may
pond temporarily.
 Avoid areas with high water tables.
1.3.0 CONSERVANCY TANK
1.3.1 What is a conservancy tank?
A conservancy tank is any covered tank without an overflow which is used for the reception and
temporary retention of sewerage and that requires routine emptying at intervals.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
1.3.2 Conservancy tank siting considerations
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Conservancy 50 feet from surface water
Conservancy tank 50 feet from all well
Conservancy tank 5 feet from foundation
Avoid low lying areas
Avoid areas where root may grow towards the conservancy tank
Every conservancy tank shall comply with the following requirements:
Such tank shall be provided with a fresh air inlet and an intercepting trap,
Such tank shall be constructed with 215mm brick or 150mm reinforced
Concrete walls on a foundation slab of mass concrete not less than 150mm thick
The tank shall be at ground level and shall be provided with one or more airtight manhole
covers to allow access to the tank for cleaning it.
The floor of the tank shall be graded to a point, which is vertically below one of the
manholes referred to in paragraph (b) and a sump not less than 300mm or more than
450mm² in plan and not less than 150mm or more than 225mm deep shall be constructed at
such point.
Such tank shall be impervious to liquid.
The owner of the property served by such tank shall provide and maintain at his own
expense a suitable road or other means of access to enable the vehicle used by the WSP to
empty such tank to reach and empty such tank.
Such owner shall pay the local WSP for the clearance of such tank in accordance with such
tariff as may from time to time be prescribed by the local WSP
No industrial, trade or manufacturing waste, refuse or effluent shall be discharged into any
conservancy tank.
1.4.0 Unplanted Drying Beds
1.4.1 What Is an Unplanted Drying Bed?
An Unplanted Drying Bed is a simple, permeable bed that, when loaded with sludge, collects
percolated leachate and allows the sludge to dry by evaporation. Approximately 50% to 80% of
the sludge volume drains off as liquid. The sludge however, is not stabilized or treated.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Feacal sludge drying beds (Source: Olufunke Cofie 2002)
The bottom of the drying bed is lined with perforated pipes that drain away the leachate. On top of
the pipes are layers of sand and gravel that support the sludge and allow the liquid to infiltrate and
collect in the pipe. The sludge should be loaded to approximately 200kg TS/m2 and it should not be
applied in layers that are too thick (maximum 20cm), or the sludge will not dry effectively. The final
moisture content after 10 to 15 days of drying should be approximately 60%. A splash plate should
be used to prevent erosion of the sand layer and to allow the even distribution of the sludge. When
the sludge is dried, it must be separated from the sand layer and disposed of. The effluent that is
collected in the drainage pipes must also be treated properly. The top sand layer should be 25 to
30cm thick as some sand will be lost each time the sludge is manually removed.
2
2 Source; Sandec/Eawag.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Structural Principle of a drying bed
1.4.2 Unplanted Drying bed siting considerations
 In regions with frequent rainfall, covering the drying beds with a roof may be considered.
 Drying beds 100 feet from surface water
 Drying beds 100 feet from all well
1.5.0 Co-composting Facility
1.5.1 What is co-composting?
Co-Composting is the controlled aerobic degradation of organics using more than one feedstock
(Faecal sludge and Organic solid waste). Faecal sludge has a high moisture and nitrogen content
while biodegradable solid waste is high in organic carbon and has good bulking properties (i.e. it
allows air to flow and circulate). By combining the two, the benefits of each can be used to optimize
the process and the product.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
3
For dewatered sludges, a ratio of 1:2 to 1:3 of dewatered sludge to solid waste should be used.
Liquid sludges should be used at a ratio of 1:5 to 1:10 of liquid sludge to solid waste. There are two
types of Co-Composting designs: open and in-vessel. In open composting, themixedmaterial (sludge
and solid waste) is piled into long heaps called windrows and left to decompose. Windrow piles are
turned periodically to provide oxygen and ensure that all parts of the pile are subjected to the same
heat treatment. Windrow piles should be at least 1m high, and should be insulated with compost or
soil to promote an even distribution of heat inside the pile. Depending on the climate and available
space, the facility may be covered to prevent excess evaporation and protection from rain. In-vessel
composting requires controlled moisture and air supply, as well as mechanical mixing. Therefore, it is
not generally appropriate for decentralized facilities. Although the composting process seems like a
simple, passive technology, a well-working facility requires careful planning and design to avoid
failure.
3Source; Sandec/Eawag.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
Co-composting facility (Open Windrow System) in Kumasi (Source IWMI, 2003)
The effluent that is collected in the drainage pipes must also be treated properly. The top sand layer
should be 25 to 30cm thick as some sand will be lost each time the sludge is manually removed.
1.5.2 Co-composting facility siting considerations
 In regions with frequent rainfall, covering the co-composting facility with a roof may be
considered.
 Co-composting facility to be 100 feet from surface water
 Co-composting facility to be 100 feet from all well
1.6.0 DEWATS
1.6.1 What Are DEWATS?
DEWATS stands for “Decentralized Wastewater Treatment Systems”. DEWATS represents a technical
approach rather than merely a technology package.
DEWATS applications are designed to be low-maintenance: most important parts of the system work
without technical energy inputs and cannot be switched off intentionally.
DEWATS applications provide state-of-the-art technology at affordable prices because all of the
materials used for construction are locally available.
 DEWATS applications provide treatment for both domestic and industrial sources
 Systems can be designed to handle organic wastewater flows from 1-1000 m3 per day
 Systems are built to be reliable, long lasting and tolerant towards fluctuations in loads
 DEWATS applications do not require sophisticated maintenance
Without considering facilities for necessary chemical pre-treatment of wastewater from industries,
DEWATS applications are designed with four basic technical treatment modules which are combined
and configured to provide a custom solution for a given sanitation/wastewater challenge:
 Primary treatment: sedimentation and floatation
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems

Secondary anaerobic treatment in fixed-bed reactors: baffled upstream reactors or
anaerobic filters
 Tertiary aerobic treatment in sub-surface flow filters
 Tertiary aerobic treatment in polishing ponds
DEWATS applications are designed and dimensioned in such a way that treated water meets
requirements stipulated in environmental laws and regulations.
Advantages of DEWATS technology:
 Provides treatment for domestic and industrial wastewater
 Low initial investment costs as no imported materials or components are needed
 Efficient treatment for daily wastewater flows of up to 1000m3
 Modular design of all components
 Tolerant towards inflow fluctuations
 Reliable and long-lasting construction design
 Low maintenance costs
Main DEWATS modules for physical and biological wastewater treatment:
1. Settler 2. Anaerobic Baffled Reactor 3. Anaerobic Filter 4. Planted Gravel Filter
1.6.2 DEWATS siting considerations
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The site size to dictate the layout of the site plan and DEWATS Modules.
Establish the availability of an area to reuse or discharge the treated waste water .
The soils MUST have the adequate depth and bearing capacity.
The topography of the area should have adequate slopes to allow flow by gravity within and
to the DEWATS.
 DEWATS should be sited above Groundwater, surface water and flood plain elevations
 Local Land use should allow the establishment of DEWATS.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
1.7.0 SUMMARY OF CONSIDERATIONS IN SITING DIFFERENT
SANITATION FACILITIES
No
1
Facility
Septic Tank
Siting Consideration
 Septic tank 50 feet from surface water
 Septic tank 50 feet from all well
 Septic tank 5 feet from foundation
 Septic tank and drain field to be 6 feet apart
 Avoid low lying areas
 Avoid areas where root may grow towards the
septic tank
2
Soak Away Pits
 Soak-away pits to be sited 50 feet from all
temporary surface water (ditches and canals)
 Soak-away to be 100,200, 0r 300 feet from
surface water (stream, lakes, or rivers)
depending on soil type
 They should be 100 feet from all wells
 Soak-away to 10 feet from foundations and 20
feet from basements
 They should be at least 5 feet from property
lines
 Areas where vehicles may compact such as
drive ways, parking and roads should be
avoided
 Steep slopes should be avoided; slopes less
than 20% are preferred.
 Avoid areas where tree may grow towards and
impact on the soak-away
 Avoid low lying areas where there are runoffs
or storm water drains or where water may
pond temporarily.
 Avoid areas with high water tables.
3
Conservancy Tanks
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Conservancy 50 feet from surface water
Conservancy tank 50 feet from all well
Conservancy tank 5 feet from foundation
Avoid low lying areas
Avoid areas where root may grow towards the
conservancy tank
Every conservancy tank shall comply with the
following requirements:
Such tank shall be provided with a fresh air
inlet and an intercepting trap,
Such tank shall be constructed with 215mm
brick or 150mm reinforced
Concrete walls on a foundation slab of mass
concrete not less than 150mm thick
The tank shall be at ground level and shall be
provided with one or more airtight manhole
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
No
Facility
Siting Consideration
covers to allow access to the tank for cleaning
it.
 The floor of the tank shall be graded to a
point, which is vertically below one of the
manholes referred to in paragraph (b) and a
sump not less than 300mm or more than
450mm² in plan and not less than 150mm or
more than 225mm deep shall be constructed
at such point.
 Such tank shall be impervious to liquid.
 The owner of the property served by such tank
shall provide and maintain at his own expense
a suitable road or other means of access to
enable the vehicle used by the WSP to empty
such tank to reach and empty such tank.
 Such owner shall pay the local WSP for the
clearance of such tank in accordance with such
tariff as may from time to time be prescribed
by the local WSP
 No industrial, trade or manufacturing waste,
refuse or effluent shall be discharged into any
conservancy tank.
4
Unplanted Drying Beds
 In regions with frequent rainfall, covering the
drying beds with a roof may be considered.
 Drying beds 100 feet from surface water
 Drying beds 100 feet from all well
5
Co-Composting Facility
 In regions with frequent rainfall, covering the
co-composting facility with a roof may be
considered.
 Co-composting facility to be 100 feet from
surface water
 Co-composting facility to be 100 feet from all
well
6
DEWATS
 The site size to dictate the layout of the site
plan and DEWATS Modules.
 Establish the availability of an area to reuse or
discharge the treated waste water.
 The soils MUST have the adequate depth and
bearing capacity.
 The topography of the area should have
adequate slopes to allow flow by gravity
within and to the DEWATS.
 DEWATS should be sited above Groundwater,
surface water and flood plain elevations
 Local Land use should allow the establishment
of DEWATS.
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Overview of Secondary Treatment Facilities Requirements and Siting of On-site Systems
1.8.0. References
1. Olufunke, C., Doulaye, K., Silke Rothenberger, Daya Moserand Chris Zurbrügg (2009) Co
Composting of Faecal Sludge and Organic Solid Waste for Agriculture: Process Dynamics.
2.BORDA;’Demand-based Technical Solutions to Reduce Water Pollution by Small and Medium
Enterprices and Settlement in Densely Populated Areas
3. http://www.grassrootswiki.org/index.php/Co-composting
4. http://www.iwapublishing.com/template.cfm?name=isbn9781780404769_
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