Monday, 21 June 2010

How to protect water supplies! (2)

Joachim Ezeji

As I discussed earlier in the month, one of the greatest challenges currently facing the water sector is how to effectively access and manage the safety of water sources in order to meet targets outlined in the Millennium Development Goals. On top of this concern is the pollution source and pathway provided by septic tank systems to water supply sources.

However, and by far, the greatest water-quality problem in developing countries including Nigeria is the prevalence of water-borne diseases, especially gastro-enteritis which is related to faecal pollution and inadequate hygiene.

A key consideration in managing a groundwater resource is its vulnerability to sources of contamination that are located primarily at and near the land surface. Because of generally low ground water velocities, once contaminants have reached the water table, their movement to nearby surface-water discharge areas or to deeper parts of the groundwater flow system is slow.

For the same reason, once parts of an aquifer are contaminated, the time required for a return to better water –quality conditions as a result of natural processes is long, even after the original source of contamination are no longer active. Groundwater quality remediation projects generally are very expensive and commonly are only partly successful.

It is therefore germane to understand what a septic tank system is, how it functions and how it can pollute groundwater. A septic tank is often a buried, water tight receptacle designed and constructed to receive wastewater from a home, to separate the solids from the liquid, to provide limited digestion of organic matter, to store solids, and to allow the clarified liquid to discharge for further treatment and disposal. The settleable solids and partially decomposed sludge settle to the bottom of the tank and gradually build up. A scum of lightweight material including fats and greases rises to the top. The partially treated effluent is allowed to flow through an outlet structure just below the floating scum layer.

This partially decomposed liquid can be disposed of through soil absorption systems, soil moulds, evaporation beds or anaerobic filters depending upon the site conditions. The most important processes that take place within the tank include separation of suspended solids, digestion of sludge and scum, stabilization of the liquid, and growth of micro-organisms. Anaerobic bacteria degrade the organic matter in the sludge as well as in the scum and as a result of this bacteria action, volatile acids are formed at the first instance and eventually are converted mostly to water, carbon dioxide and methane. The formation of gases in the sludge layer causes irregular flotation of sludge flocs that resettle after the release of the gas at the surface.

The performance of a septic tank greatly depends on its design. A properly designed septic tank performs efficiently in the removal of settleable matter and the Biochemical Oxygen Demand (BOD).However, the effluent from a septic tank still contains high concentrations of BOD, pathogens, nitrogen and phosphorus, which prohibits its discharge into any water course or on land without further treatment.

Under normal design conditions, reductions in BOD of 25-50% and in suspended solids (SS) of up to 70% have been reported in literature. The high reduction in BOD and SS can however be obtained by prolonging the retention time, which in most cases may not be practicable. Apart from the retention time, the other factors which affect the performance of the septic tank are; ambient temperature, the nature of the influent waste water, the organic content, the positions of the inlet and outlet devices in the tank etc.

The digestion of the sludge and scum depends on the microbial population and the temperature. Sludge and scum decompose more slowly at lower temperature and are accelerated by an increase in temperature.
The effluent from a septic tank is only partially treated and still contains high concentration of micro-organisms, BOD, phosphorus and nitrogen, which should not be discharged directly into a public water course or on land. Further treatment or other means of disposal are required. Where site conditions are suitable and do not pose any threat to Groundwater quality, sub-surface soil absorption is usually the best method for septic tank effluent disposal.

However, the performance of the soil absorption systems depends on the ability of the soil to accept liquid, absorb viruses, strain out bacteria and filter the waste. A proper site evaluation requires accurate measurement of the soil permeability, the degree of slope, the position of the water table and the soil depth.

The following general guidelines can be considered for selecting soil absorption sites; soil permeability should be moderate to rapid and the soil percolation rate should be generally 24 minutes per cm or less. The Groundwater level during the wettest season should be at least 1.22m (4ft) below the bottom of the sub-surface absorption field or soak pit. Impervious layers should be more than 1.22m below the seepage bed or the pit bottom.
The site for an absorption field of a soak pit should not be within 15.24m (50ft) of a stream or other water body.
A soil absorption system should never be installed in an area subject to frequent flooding.

Three different types of sub-surface soil absorption systems are commonly used; absorption trenches; absorption beds or seepage beds; and absorption pits or soakage pits. The use of these types depends on the suitability of soil and other local conditions. These are deep excavations used for sub-surface disposal of septic tank effluent. Absorption pits are recommended as an alternative where absorption fields/trenches are not practicable and where the topsoil is underlain with porous soil or fine gravel. The capacity of an absorption pit can be computed on the basis of percolation tests to be made at the disposal site.

Soakaways or soakage pits are mostly used in urban Nigeria but more troubling in densely populated areas. The septic tank effluent flows through pit walls made of open jointed bricks, into the surrounding soil. Typically, soakaways can be 2 to 3.5m in diameter, and 3 to 6m deep depending on the amount of wastewater flow and the infiltration capacity of soil. Leach pits for VIPs and pour flush latrines have to be designed for storage and digestion of excreted solids as well as infiltration of the liquid waste into the surrounding soil. Designing for storage and digestion of solids is exactly the same as for all dug pit latrines.

Infiltration of the liquid effluent however, requires that sufficient pit-soil interface area is available depending on the long term infiltration capacity of the soil. Pit effluent enters the soil first by infiltrating the pit-soil interface, which is partially covered in a bacteria/slime layer, and then by percolating away through the surrounding soil. The long term infiltration rate depends on the type of soil. The liquid effluent from leach pits of different types of pit latrines will infiltrate both laterally and vertically into the soil and through to the groundwater if the aquifer is an unconfined one.

The basic principles of on-site systems, however, remain the same: liquids infiltrate into the soil and the solids are retained, anaerobically digested and have to be removed or a new pit has to be dug at regular intervals. The basic on-site systems are primarily designed to dispose of human excreta. Wastewaters from cooking, clothes washing, and bathing are collected in small drains and disposed of in soakaways for infiltration.

If the leach pit bottom is close to the Groundwater table, then bacteria or other contaminants may travel both downwards and laterally, transported by the Groundwater. The lateral movement will always be in the same direction as the flow of the Groundwater. It is therefore important that latrine locations are carefully selected with respect to sources of water supply, to avoid the risk of pollution.

It has been indicated that if there are at least 2.0meters between the pit bottom and the Groundwater table, little microbial pollutant travel occurs in most unconsolidated soils and a horizontal distance of 10.0meters between a drinking water well and a latrine is often satisfactory.

Concluded !

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