Sewage treatment

This section suggests that it is often best to connect to the mains drainage system if available. It goes on to outline the options for on-site treatment if the site is not served by mains drainage and also outlines radical alternatives which can be used.

Mains drainage

About 96% of the population is connected to a main drainage system. This carries sewage to a treatment plant which transforms it into more or less clean water and sludge. It is generally the safe, convenient, cost-effective and least energy-intensive option. The process is monitored and the standards

required for discharges of effluent are rising under the influence of, among other things, the EU Urban Waste Water Treatment Directive. It is often best to connect to the main drainage system if it is available, and you may be obliged to do so if the site is served by mains drainage. However, one alternative is to treat the sewerage on site and return the cleaned water to the soil or into a watercourse. This is fine if the system is designed, constructed, monitored and maintained correctly.

On-site treatment

Of the 4% of the population not served by mains drainage, most use a septic tank and soakaway that disposes of the tank effluent into the soil. When this is not possible due to soil conditions or lack of space, there are several methods of making the waste-water clean enough to discharge to a watercourse. The appropriate system is the one that causes the least environmental impact when viewed from a lifecycle perspective. It may not be the solution that is apparently the most ‘green’.

There are many techniques for on-site waste- water management. It is important to understand their various merits and drawbacks so as to pick the most appropriate technique for a particular circumstance. There is a risk that systems may not perform to the required ecological standards, which will be a problem for both residents and the environment.

Appropriate solutions are very site-specific. If a small volume of effluent is to be discharged to a large river (or to the ground), the level of appropriate treatment will be far less stringent than when a large volume of effluent is to be discharged to a small stream or close to a groundwater supply. Higher levels of treatment than appropriate usually incur environmental (and financial) costs that may offset the assumed ecological benefit of a purer effluent.

Sewage treatment

Most systems rely on a primary treatment stage of physical settlement to remove gross matter followed by a secondary biological treatment using aerobic micro-organisms. This can sometimes be followed by a tertiary stage that further clarifies the effluent and removes nutrients and pathogens. The following options can be designed in different combinations to suit the particular circumstances and use.

Primary treatment options include:

  • septic tank in which the solids and up to one half of the organic load are retained as crust or sludge. This is the most common installation, it is simple and economical, but requires the sludge to be emptied (usually every couple of years)
  • packaged units generally include secondary treatment. They are a compact, readily available, medium-cost alternative with minimum maintenance often carried out by the installer under contract. They do use power, and no treatment is

possible if there is power or mechanical failure. There are several alternative types including the Rotary Biological Contactor (RBC) which has a series of large-surface-area plastic discs half submerged in the effluent. They rotate slowly, powered by a motor, so exposing to air a film of micro-organisms that builds up on the discs; this creates the conditions for aerobic treatment. Another type is the Sequencing Batch Reactor (SBR) which aerates a batch of incoming sewage with air in a single chamber, then lets the sludge settle out and finishes the sequence by drawing off the effluent.

Secondary treatment options include:

  • Percolating filter This is an open tank filled with clinker or stones. The effluent passes over these, forming a thin film. This exposes to air the micro-organisms flourishing in it, allowing aerobic treatment. Clumps of micro-organisms must then be cleared from the effluent in a settlement tank. Circular versions with a rotating arm to distribute the effluent are a common sight in small sewage-treatment works. This is a tried and tested technique that requires little power, although it is relatively expensive. It is not generally appropriate for a small number of homes.
  • Reedbeds A vertical-flow reedbed is similar to a percolating filter but with a layer of sand on top, planted usually with common reeds. A following settlement tank or second reedbed are usually needed. Reedbeds achieve a high level of treatment without the use of power, and maintenance is straightforward but does require awareness of the processes. It needs more space than some other methods, around 12m2 per family home, and is relatively costly. The specification of the sand and the size of the bed are critical to prevent blockage. A horizontal-flow reedbed is shallower and designed to be full of water, unlike the vertical-flow type which is free-draining. The effect is to reduce the amount of oxygen, creating ideal conditions for the removal of nitrogen from treated effluent. A horizontal-flow reedbed is therefore best used at a tertiary stage, where it also removes pathogens. It can be about half the size of a vertical-flow bed, which is used for secondary treatment. Opinions vary on the role played by the reeds, but reedbeds almost certainly are not the single answer to sustainable sewage treatment, despite their iconic status.
  • Ponds can be used for primary, secondary and tertiary treatment. They are usually seeded with aquatic plants and can look beautiful. A large surface area is required, around 50 to 100m2 for a family house, and are relatively expensive. Aerated ponds require less area and should be odour-free, but typically use more electrical energy than package plants such as RBCs.
  • Leachfields These commonly follow a septic- tank primary stage. A leachfield is a series of perforated pipes in trenches surrounded by gravel. This can be completely unseen below a lawn. It provides good-quality treatment at low cost and low maintenance. It is natural treatment in its simplest and most efficient form and is the preferred option for smaller on-site sewage-treatment systems where the soil is sufficiently free-draining to allow effluent to drain into the soil.
  • Living machines is the name coined for an artificial ecosystem created with planting in a glasshouse and intended to imitate nature in the treatment of effluent. However, living machines are expensive to build and operate. They require power for pumps and aeration, need close management and operate at low efficiency. They are not ‘green’ from the viewpoint of environmental impact.

 

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