GROUNDWORKS 37
MINIMISING WASTE IN WATER TREATMENT
With new regulations governing the use of septic tanks in force, David Stagg of Graf UK talks housebuilders and developers through their options for compliance.
H
ousebuilders and developers building properties off the mains sewage network need to be
aware of a recent change – a bid by the Environment Agency to reduce the level of pollution from sewage in the nation’s watercourses. Under the General Binding Rules, anyone with a septic tank discharging into a watercourse must have plans in place to replace it or upgrade the foul water solution within a reasonable timescale, typically 12 months, or sooner if the property is sold before this date. The rules are as follows:
• Use the correct treatment system – a septic tank or small sewage treatment plant – so the Environment Agency can assess the risk of using it in the location • The treatment plant must meet British Standard BS EN 12566 • The treatment plant must be installed correctly and have enough capacity • The treatment system must be regularly emptied and maintained
• If the property is sold, the new owner must be informed about the sewage treatment system
• If use of the treatment system ceases, it must be properly decommissioned.
How can existing septic tanks be upgraded? There are four options avail- able to specifiers: • If a mains sewer has become available, connect to it
• If space is available, install a drainage field so the septic tank discharges into the ground
• Replace the septic tank with a small sewage treatment plant • In exceptional circumstances, a permit can be applied for to allow discharge to surface water.
To establish what you should do to ensure a sewage treatment system meets the “new” rules, consider the following. If the existing septic tank discharges to
a drainage field that is correctly sized for the tank, then it is compliant, but if it is not, then the correct size needs clarifying, and equipment modified to
meet the requirement. If the existing septic tank discharges
to a soakaway, watercourse, river or stream and there is space for a drainage field, clarify the size of drainage field required and install it as the discharge point from the septic tank. If there is not enough space for a
drainage field, then the septic tank must be upgraded to a sewage treatment plant. If the sewage treatment plant cannot discharge into a local watercourse then a permit needs to be applied for it to discharge to a soakaway. So, what is all this going to cost?
Well, the lowest cost long-term is most likely to be to connect to a mains sewer if one has become available. The next best cost option is to install
a new sewage treatment plant, which is likely to start at somewhere around £2,000, based on British guidelines that a three-bedroomed house would need a minimum of a five-person treatment plant. Installation would be on top of this and varies greatly but could add a further £5,000 to £10,000. A higher cost would be to install a
drainage field if the land required is available and the soil conditions are suitable. However, depending on its size, this may be similar to the cost of a new sewage treatment plant as many metres
of perforated pipe might be needed, which can mean a significant area of land has to be excavated. It is difficult to put a sum on this as it depends on the percolation rate of the soil, but it is not likely to be a small area to dig up, and therefore the labour and plant hire alone is likely to cost more than some of the earlier options. Infiltration tunnels are a great alternative to traditional slotted pipe (that takes up a great deal of land space), but these will require a permit from the Environment Agency. Last but not least is the question of what to look for in a sewage treatment plant. Specify one which controls effluent quality discharge, while being efficient on energy consumption through the use of a sequence batch reactor process. Look for one which can automatically adapt to a system becoming under or over-charged so performance is kept to a high standard and energy efficiency is not compromised. In addition, choose one with a treat-
ment process that results in a high-quality effluent discharge that far exceeds current
regulations and is future-proofed for any regulation tightening (the British Standard of 20 mg/l for biochemical oxygen demand, 30 mg/l for suspended solids and 20 mg/l for ammonia nitro- gen). This means that under normal circumstances effluent can be discharged directly into a flowing watercourse – although special environments, such as Areas of Outstanding Natural Beauty, may require a higher quality of treat- ment. Find one with discreet manhole covers, that does not require concrete backfill, and has no electrical components within the tank as well as a long sludge storage period. Then, not only will your clients be
compliant, they will also be certain of a cost-effective, low-maintenance future in wastewater treatment.
David Stagg is technical product specialist for Graf UK
WWW.HBDONLINE.CO.UK
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