In many respects, the commonest rural solution to sewage treatment beyond the reach of sewerage, namely the septic tank, makes use of an intermediate form of land treatment. In the so-called cesspit, a sealed underground tank, collects and stores all the sewage arising from the household. At regular intervals, often around once a month dependent on the capacity, it requires emptying and tanker-ing away, typically for spreading onto, or injection into, agricultural land. By contrast, a septic tank is a less passive system, settling and partially digesting the input sewage, although even with a properly sized and well-managed regime the effluent produced still contains about 70% of the original nutrient input. In most designs, this is mitigated by the slow discharge of the liquid via an offtake pipe into a ground soakaway, introducing the residual contaminants into the soil, where natural treatment processes can continue the amelioration of the polluting constituents. There are various types of septic systems in use around the world, though the most common, illustrated in Figure 6.1, is made up of an underground tank, which is linked to some form of in situ soil treatment system, which usually consists of a land drainage of some kind.
Since a system that is poorly designed, badly installed, poorly managed or improperly sited can cause a wide range of environmental problems, most espe-cially the pollution of both surface and groundwaters, their use requires great care. One of the most obvious considerations in this respect is the target soil’s ability to accept the effluent adequately for treatment to be a realistic possibility and hence the percolation and hydraulic conductivity of the ground are important factors in the design and long-term success of this method.
Under proper operation, the untreated sewage flows into the septic tank, where the solids separate from the liquids. Surfactants and any fat components tend to float to the top, where they form a scum, while the faecal residues remaining after bacterial action sink to the bottom of the tank, to form a sludge. The biodegradation of the organic effluent in these systems is often only partially complete and so there tends to be a steady accumulation of sediment within the tank, necessitating its eventual emptying. This settling effect produces a liquid phase which is permitted to flow out of the tank, along an overflow pipe situated towards the top of the vessel and is discharged to the soil as previously described. Internal baffles inside the tank are designed to retain the floating scum layer and prevent undegraded faeces from leaving the system prematurely. If these biosolids were permitted to wash out into the soil its ability to treat the septic-tank effluent can readily become compromised, leading to a reduction in the overall system efficiency.
The drainage arrangements associated with a septic tank system are, arguably, perhaps the most important part of this whole approach to sewage treatment and may be considered as effectively forming an underground microbiological processing plant. Clearly, it is of vital importance that the soil on any given site must be suitable for the drainage to function reliably. The only way to be certain is, of course, by means of a percolation test, though as a general rule, clay soils are unsuited to this purpose. In circumstances of defined clay strata, particularly when they exist close to the surface, it is highly unlikely that straightforward drainage arrangements will prove satisfactory. Even in the absence of a high clay content, soils which are either too fine or very coarse can also reduce the effectiveness of this phase of the treatment system. The former can be a problem because, like clay, it also resists effluent infiltration, the latter because it permits it too quickly and thus retention time becomes inadequate for the level of treatment needed.
A further consideration which must be addressed in this respect is the position of the water table, which may cause problems for the drainage system if it lies within half a metre of the surface. Consequently in areas where this is a permanent or even seasonal feature, the drains may be established much higher than would be typical, frequently in close proximity to the soil surface. This brings its own inevitable set of concerns, not least amongst them being that there can be a very real possibility of the relatively untreated effluent breaking through to above ground.
One solution to this potential problem that has been used with some success is the sewage treatment mound. Formed using clean sand or small gravel, the mound elevates the system so that it sits a metre or so above the level of the seasonal highest water table. The construction of the mound needs to receive careful consideration to produce a design which suits the local conditions, while also guaranteeing an even distribution of the septic tank effluent throughout the mound. Typically, these systems are intermittently fed by a pump from a collec-tion point and the rate at which the liquid off-take flows through the soil is a critical factor in the correct sizing of the drainage mound. In the final analysis, the sizing of all septic tank systems, irrespective of the details of its specific design, depends on the amount of sewage produced, the type and porosity of soil at the site and the rate at which water flows through it. Proper dimensional design and throughput calculations are of great importance, since the efficacy of septic systems is readily reduced when the set-up is overloaded.
Most modern installations use premanufactured tanks, typically made of stable polymer and formed in a spherical shape with a short shaft like the neck of a bottle forming a ground level inspection point. They often have a series of internal baffles moulded within them to facilitate the flow of liquids and retention of solids and surface scum, together with the appropriate pipework inlets, outlets and gas vents. This type of tank has become increasingly popular since they are readily available, easier to site and can be operational much faster than the older concrete designs.
The most common versions of these consisted of two rectangular chambers which were originally built out of brick or stone until the advent of techniques to cast concrete in situ. Sewage digestion was incompletely divided into two stages, with gas venting from the primary chamber and secondary also, in better designed systems. These were sometimes associated with an alternative soil-dosing phase, known as seepage pits and soakaways, in which the part-treated effluent arising from the septic tank is discharged into a deep chamber, open to, and contiguous with, the soil at its sides and base. This permitted the free translocation of liquid from the seepage pit into the surrounding soil, the whole of the surrounding ground becoming, in effect, a huge soakaway, allowing dilution and dispersal of the effluent and its concomitant biotreatment within the body of the soil. In practice, provided the character of the ground is truly suitable for this approach, effluent infiltration and remediation can be very effective. How-ever, if the soil porosity precludes adequate percolation, the potential problems are obvious.
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