lt_vault.wmf

The latrine in conjunction with a vault is used to collect >nightsoil on-site. Excreta fall through the latrine hole into a watertight tank (vault). The vault is emptied in regular intervals (e.g. every fortnight) by pumping the contents into a tanker truck, which transports it to a disposal site or a nightsoil treatment plant.

One unit can serve one or several households.

This facility can not receive >greywater.

Cartage systems are applicable to very high population densities.

Vault construction requires skilled labour. Construction of the superstructure offers potential for >self help.

Apart from regular emptying, no further maintenance is required.

If vaults are constructed properly and emptied using adequate equipment (i.e. hose and vacuum pump, no manual scraping), then the standard of hygiene provided is good.

Contrary to the >pour-flush toilet and vault system, excreta are visible through the latrine hole, and the absence of a water seal promotes odours and insect nuisance.

The >pour-flush toilet in conjunction with a vault is used to collect >nightsoil on-site. Excreta are flushed into a watertight tank (vault). The vault is emptied in regular intervals (e.g. every fortnight) by pumping the contents into a tanker truck, which transports it to a disposal site or a nightsoil treatment plant.

One unit can serve one or several households.

Cartage systems are applicable to very high population densities.

Vault construction requires skilled labour. Construction of the superstructure offers potential for >self help.

Apart from regular emptying, no further maintenance is required.

If vaults are constructed properly and emptied using adequate equipment (i.e. hose and vacuum pump, no manual scraping), then the standard of hygiene provided is good.

Simple construction results in a high potential for >self help.

Public pour-flush toilets discharge into a >septic tank which is usually located underneath them.

For cost estimating, SANEX assumes that one toilet block consists of no more than ten toilet units, each serving 25 persons.

The construction of public facilities requires skilled contractors and labour. There is little potential for >self help.

The auqaprivy is designed for the on-site collection and treatment of domestic sewage. Excreta fall through a submerged chute into a watertight tank, which is located underground. The liquid in the tank provides a water seal to reduce odours and insect nuisance.

One unit can serve one or several households.

There are two main treatment effects:

1) Contaminants are removed from the sewage by either the settling of heavy particles or by flotation of materials less dense than water (e.g. oils and fats). The sludge layer at the bottom of the tank is a result of the settling process. The scum layer is formed through the flotation process.

2) Subsequently, organic matter in the sludge as well as the scum layer is digested by bacteria. As a result, gas is produced which emerges through a ventilation opening in the tank. The digestion process is important because it prevents the excessive accumulation of sludge.

Skilled labour is required for the construction of aquaprivies. There is some potential for >self help.

Accumulated sludge needs to be pumped out every one to three years, which requires special equipment such as vacuum trucks. Besides cleaning the chute, there is no other maintenance required.

For flushing and in order to maintain the water seal, approximately five litres of water is required per person per day.

If not well constructed, tanks are not watertight. As a consequence, it is difficult to maintain the liquid level and thus the water seal, with resulting bad odours.

drainfld.wmf

The drain field is designed for the on-site disposal of sewage. It is an area of land consisting of one or several long trenches, into which sewage is discharged through underground perforated pipes. The sewage percolates into the ground while it is decomposed by bacteria in the soil.

Usually, one drain field receives the effluent from one >septic tank or aquaprivy.

The construction of drain fields is simple with good potential for >self help.

Large space requirements are often a problem. The size and the cost of drain fields depend on the absorption capacity of the soil.

If constructed properly, no maintenance is required. But if clogged, replacement trenches are required.

Open defecation occurs where there are no toilet facilities, or available facilities are objectionable to dwellers.

The chances that people or (farm) animals come into contact with excreta are high. The result is an increased risk for pathogen transmission.

The situation is especially unhygienic in areas with moist soil and surface water run-off.

The pit latrine is designed for the on-site disposal of human excreta. It consists of a concrete squatting plate which is placed over an earthen pit. Its design life is between 15 - 30 years. If less than 10 years, a >double vault composting (DVC) latrine should be considered instead.

The pit diameter is between 1 - 1.5 m. The depth of the pit is at least 2 m, but usually more than 3 m. The top 0.5 m of the pit always require lining. In loose soil, the entire pit should be lined in order to prevent collapse.

One unit can serve one or several households.

Pit latrines do not need water for flushing and are simple to construct. Thus the potential for >self help is high. If constructed properly, they provide good health benefits.

All types of anal cleansing materials may be used.

Since ventilation of pit latrines is simple, odours and insect nuisance may occur. Excreta can be seen through the hole in the squatting plate.

Since pit latrines involve soil absorption, there is a danger of groundwater contamination.

The pour-flush latrine is designed for the on-site disposal of human excreta. Its construction is similar to that of the >pit latrine, except that it uses a >pour-flush pan instead of a squatting plate with a hole in it.

One unit can serve one or several households.

Pour-flush latrines need small amounts of water for flushing.

They are simple to construct. Thus the potential for >self help is high. If constructed properly, they provide good health benefits.

The water seal in the >pour-flush pan forms an effective barrier against odours and insect nuisance. Excreta are invisible.

Since pit latrines involve soil absorption, there is a danger of groundwater contamination.

The Reed odorless earth closet (ROEC) is designed for the on-site disposal of human excreta. From the concrete squatting plate, an inclined chute leads to the completely off-set pit. Ventilation is similar to the >VIP latrine. Its design life is between 15 - 30 years. If less than 10 years, a >double vault composting (DVC) latrine should be considered instead.

Because it is off-set, the pit can be built larger than that of the conventional >pit latrine. In loose soil, the entire pit should be lined in order to prevent collapse.

One unit can serve one or several households.

ROECs do not need water for flushing and are simple to construct. Thus the potential for >self help is high. If constructed properly, they provide good health benefits.

All types of anal cleansing materials may be used.

Good ventilation largely prevents odours and insect nuisance.

Since the chute is inclined, excreta cannot be seen through the hole in the squatting plate. However, fouling of the chute is often a problem.

Since ROECs involve soil absorption, there is a danger of groundwater contamination.

The seepage pit is designed for the on-site disposal of sewage. It consists of an underground pit, the wall of which is lined with bricks. Through the open joints in the brick lining, sewage percolates into the soil, where it is decomposed by microorganisms.

Usually, one seepage pit receives the effluent from one >septic tank or aquaprivy.

The construction of drain fields is simple with good potential for >self help.

Large space requirements are often a problem. The size and thus the cost of a seepage pit depends on the absorption capacity of the soil.

If constructed properly, no maintenance is required.

The septic tank is designed for the on-site treatment of domestic sewage. The tank is located underground and usually consists of two compartments. The first compartment is approximately twice as large as the second one. Septic tanks can be constructed with only one compartment. However, this will result in significantly reduced treatment effects and cost savings are minimal.

There are two main treatment effects:

1) Contaminants are removed from the sewage by either settling of heavy particles or by flotation of materials less dense than water (e.g. oils and fats). The sludge layer at the bottom of the tank is a result of the settling process. The scum layer is formed through the flotation process.

2) Subsequently, organic matter in the sludge as well as the scum layer is digested by bacteria. As a result, gas is produced which emerges through a ventilation opening in the tank. The digestion process is important because it prevents the excessive accumulation of sludge.

Septic tanks can reduce the >BOD of raw sewage by up to 40% and the >suspended solids content by 65%. Their effluent is thus much more readily absorbed into the ground than raw sewage. Therefore, smaller soil absorption facilities (e.g. >seepage pit or >drain field) are required.

The construction of septic tanks requires skilled labour, there is some potential for >self help.

Depending on their design, septic tanks require routine checks for sludge and scum levels and desludging every one to three years. If this maintenance is neglected, septic tanks produce very poor effluent and can become a serious environmental and health hazard.

Since they can only accept liquid waste, they must be connected to a flush toilet. Thus they are not suitable where water supply is scarce or unreliable.

The effluent of septic tanks is still heavily contaminated with pathogens. Therefore, its disposal requires either soil absorption facilities or sewerage. An exception to this are >septic tanks for excreta reuse.

A trench latrine consists of an unlined shallow pit or trench which is used for defecating. When almost full, it is refilled and sealed with soil.

The ventilated improved pit (VIP) latrine is designed for the on-site disposal of human excreta. With the exception of some enhancements in design (e.g. a vent pipe) to improve ventilation, its construction is similar to that of the >pit latrine.

One unit can serve one or several households.

VIP latrines do not need water for flushing and are simple to construct. Thus the potential for >self help is high. If constructed properly, they provide good health benefits.

All types of anal cleansing materials may be used.

Since the ventilation of VIP latrines is good, odours and insect nuisance normally do not occur.

Excreta can be seen through the hole in the squatting plate.

Since VIP latrines involve soil absorption, there is a danger of groundwater contamination.

The biogas digester is an on-site facility which produces biogas (mainly methane) that can be used for cooking and lighting. Since the technology does not work well with human excreta alone, it is only suitable for rural areas where large animals (e.g. pigs or cattle) are held.

The rather large fermentation tank (approximately 10 cubic metre for a single household) is fed with human excreta (e.g. from a >pour-flush toilet), diluted animal faeces, and crop stalks.

A well designed and operated digester produces enough gas to cover the needs of an entire household. Because of the long liquid retention time, the effluent slurry may be safely used as fertilizer in agriculture and aquaculture.

>Greywater can be used to dilute animal faeces. However, for costing purposes SANEX assumes separate collection and disposal of greywater.

Skilled labour is required for the construction of a watertight tank, with little scope for >self help.

Significant amounts of water are required to dilute animal faeces.

Biogas production falls off at low ambient temperatures. It is thus advantageous to bury the tank. Biogas digesters are not suitable for cold climates (i.e. winter temperatures significantly below zero for a prolonged period of time).

The double vault composting (DVC) latrine is designed for the on-site collection and treatment of excreta. It is similar to the >pit latrine. However, it consists of two smaller pits which are located side by side, each of which holds the excreta volume of one year. The pits are used alternatingly. This means that the useful life of a DVC latrine is virtually unlimited.

After the period of one year, compost is free of pathogens and safe for reuse (optional).

DVC latrines can be used in areas with high population density.

Their construction is simple and can be done by unskilled labour (i.e. good potetntial for >self help).

To support the composting process, easily biodegradable wastes such as grass, vegetable and fruit waste must be added to the operating vault. Other maintenance required is the emptying of one pit each year.

If ventilated similar to the >VIP latrine, no odours or insect nuisance occur.

Composted pit contents are usually easier to excavate than a new pit (as it would be necessary for the >pit and >VIP latrines).

If not equipped with a pour-flush toilet, but only with a squatting plate with a hole in it, all types of anal cleansing materials may be used.

Since DVC latrines involve soil absorption, there is a danger of groundwater contamination.

The septic tank for excreta reuse (STER) is designed for the on-site treatment of human and animal excreta. Though it still contains significant amounts of pathogens, the slurry from the storage tank can be used as fertilizer in agriculture and aquaculture.

The STER is located underground and usually consists of three compartments. These are much larger than the two compartments of a conventional >septic tank, thus providing long retention times for effective digestion and removal of pathogens.

There are three main treatment effects:

1) Contaminants are removed from the sewage by either settling of heavy particles or by flotation of materials less dense than water (e.g. oils and fats). The sludge layer at the bottom of the tank is a result of the settling process. The scum layer is formed through the flotation process.

2) Subsequently, organic matter in the sludge as well as the scum layer is digested by bacteria. As a result, gas is produced which emerges through a ventilation opening in the tank. The digestion process is important because it prevents the excessive accumulation of sludge.

3) Long retention time reduces the concentration of pathogens to acceptable levels.

>Greywater can be used to dilute animal faeces which are fed directly into the second compartment. However, for costing purposes SANEX assumes separate collection and discharge of greywater.

The construction of STERs requires skilled labour, there is some potential for >self help.

STERs require routine checks for sludge and scum levels and desludging every two years. If this maintenance is neglected, STERs produce an effluent slurry which from a hygiene point of view might be unacceptable.

Fish ponds are shallow earthen basins used for fish (mostly carp and tilapia) breeding. Ponds are fertilized with human or animal excreta which are decomposed by the micro- and macrobiotic organisms in the pond. In good conditions, yearly yields of fish can be as high as several hundres kilograms per hectare.

Due to large space requirements, fish ponds are only feasible in rural areas where people do not object to fish reared in excreta enriched water. SANEX assumes that the land cost in areas where fish ponds are to be built is negligible.

SANEX considers fish ponds feasible only for small communities with a population size of up to 1000 persons. The maximum treatment capacity is 500 persons per hectare of pond area. Above the population size of 1000 persons, ponds might still be desirable for fish cultivation. However, as a sole treatment and disposal facility for the entire community, they are considered insufficient. Complementary facilities of another type would be required.

The toilet bowl consists of a siphon, which provides a water seal against bad odours from the effluent pipe. Excreta are flushed away with water stored in the cistern (depending on the type between five to twenty litres per flush).

Cistern-flush toilets provide the highest level of convenience and have a very clean and hygienic appearance.

Cistern-flush toilets use large amounts of water. SANEX assumes that installing them results in a water use of around one hundered litres per person per day.

The toilet pan consists of a siphon, which provides a water seal against bad odours from the effluent pipe. Excreta are flushed away with water which is poored manually into the pan by using a scoop. The amount of water required to flush this type of toilet is between two and three litres.

Pour-flush toilets provide a high level of convenience and have a very clean and hygienic appearance.

Sewage from toilets flows in a system of underground pipes to treatment facilities or directly into receiving waters.

Conventional sewerage consists of house connections to a reticulation sewer system. Reticulation includes pumping stations and rising mains to the trunk sewer system. Manholes provide access to critical pipe sections (e.g. bents, joints, connections) for maintenance and cleaning.

The number of pumping stations, manholes and required pipe diameters (and thus the cost of the system) largely depends on the demographic and topograhic characteristics of the area.

SANEX assumes that costs of a covered stormwater drain system are approximately 25% of what >conventional sewerage would cost. The difference to >simplified sewerage (35%) is not large because drains receive not only sewage but also stormwater. Thus, the capacity of the trunk system needs to be larger.

In order to allow stormwater to enter the system, periodic inlets in the drain covers are required. These will emit odours, especially in sections where the flow velocity is slow.

Covered stormwater drains are not suitable in areas where flooding occurs.

Skilled engineers, contractors and labour are required for construction as well as maintenance. There is no >self help potential.

Settled sewerage receives the effluent from >septic tanks, where most settleable solids are removed, so that only the liquid part of raw sewage enters the sewerage system.

In principle, the design of a settled sewerage network is similar to that of >conventional sewerage. However, since no self-cleansing flow velocity is required, pipes with a smaller diameter can be laid at much flatter gradients, which results in less excavation.

Settled sewerage is prone to malfunctioning if septic tanks are not designed and maintained properly (i.e. desludged regularely). This problem can become particularely difficult if no single entity (e.g. agency) is responsible for the whole system (i.e. septic tank maintenance is the task of the household and sewerage maintenance is the task of the municipality).

Simplified sewerage works like >conventional sewerage. However, design criteria for its construction are far less conservative. They are selected to just comply with hydraulic minimum requirements. Massive cost savings result from:

- Smaller pipe diameters

- Laying pipes at flatter gradients

- Laying pipes at shallow depth

With regard to self-cleansing and capacity, pipes in a settled sewerage system operate closer to their hydraulic limit. This results in a somewhat higher risk for operating problems (e.g. clogging).

Sewage from toilets flows in a system of underground pipes to treatment facilities or directly into receiving waters.

Conventional sewerage consists of house connections to a reticulation sewer system. Reticulation includes pumping stations and rising mains to the trunk sewer system. Manholes provide access to critical pipe sections (e.g. bents, joints, connections) for maintenance and cleaning.

The number of pumping stations, manholes and required pipe diameters (and thus the cost of the system) largely depends on the demographic and topograhic characteristics of the area.

Large septic tanks work similar to individual >septic tanks, except that they serve multiple households, receiving effluent from a sewerage network.

Generally, they are divided into three compartments or use two parallel tanks (as illustrated).

SANEX assumes that the communal septic tank is the final treatment stage (instead of e.g. >activated sludge treatment) in a process train. Thus, its treatment efficiency is equivalent to >primary treatment, with removal rates of over 40% of >BOD and around 65% of >suspended solids. Pathogen removal is poor, and effluent is very hazardous from a health point of view.

The construction of communal septic tanks requires engineers and skilled contractors.

Communal septic tanks require routine checks for sludge and scum levels and desludging every year. If this maintenance is neglected, they produce very poor effluent and can become a serious environmental and health hazard. Other than desludging, they do not require significant maintenance. There is little potential for >self help.

Imhoff tanks serve multiple households, receiving effluent from a sewerage network. They work similar to >communal septic tanks.

Sedimentation of solids occurs in the upper settling compartments. Sludge falls through the slot at the bottom of the settling compartment into the lower tank, where it is digested anaerobically. This process generates methane gas which, to prevent it from disturbing the settling process, is deflected by the baffles to the gas vent channels.

SANEX assumes that the Imhoff tank is the final treatment stage (instead of e.g. >activated sludge treatment) in a process train. Thus, its treatment efficiency is equivalent to >primary treatment, with removal rates of over 40% of >BOD and around 65% of >suspended solids. Pathogen removal is poor, and effluent is very hazardous from a health point of view.

The construction of Imhoff tanks requires engineers and skilled contractors.

Imhoff tanks require wasting of sludge and removal of scum at regular intervals. If this maintenance is neglected, they produce very poor effluent and can become a serious environmental and health hazard. Other than desludging, they do not require significant maintenance. There is little potential for >self help.

Primary treatment (PT) is a train of processes designed to treat wastewater collected from a sewerage network.

Preliminary treatment removes coarse solids and grease. Settling in the clarifier further removes solids. The clear supernatant in the clarifier can be discharged or treated further (e.g. >activated sludge treatment). Sludge which accumulates at the bottom of the clarifier needs to be drained and disposed off.

Removal rates of over 40% of >BOD and over 60% of >suspended solids can be expected.

The implementation of primary treatment facilities requires skilled engineers, contractors and labour.

Operation and maintenance require trained staff, and, depending on the technology used, spare parts and other commodities which can be hard to obtain in remote or undeveloped areas.

Waste stabilization ponds (WSP) are a sequence of three or more ponds, designed to treat wastewater collected from a sewerage network.

The anaerobic pond receives the raw sewage. It is comparatively small and deep. Organic pollutants and sludge building up at the bottom of the pond are digested by anaerobic microorganisms. A scum layer from fats and grease which gradually forms at the pond surface prevents the pond from being aerated (e.g. by wind) and turning aerobic.

The facultative pond receives the pretreated effluent from the anaerobic pond. It is shallower with a large surface area and consists of an aerobic zone close to the pond surface as well as a deeper anaerobic zone.

The maturation pond receives the low strength effluent from the facultative pond and polishes it to degree which can fulfill very stringent effluent requirements. It plays an important role for pathogen removal. The maturation pond is very shallow with a large surface area. It is aerobic with oxygen being transfered into the water by wind and algae (photosynthesis).

With regard to the removal of organic pollutants, a well designed WSP system performs as well as >activated sludge treatment. SANEX considers ponds a low-tech alternative to activated sludge treatment, and thus assumes that there is no effluent disinfection (e.g. chlorination). This assumption leads SANEX to reject the WSP system as an alternative to activated sludge treatment if an effluent >fecal coliform count of less than 1000 MPN/100 mL is required.

The construction of WSPs requires skilled engineers and contractors.

The operation and maintenance of WSPs is very simple and only requires unskilled labour.

WSPs can produce a very clean effluent which is equivalent to that of >activated sludge plants. After a couple of years of operation, the removal of sediments can become necessary.