Green by name, green by nature
The greenwater treatment plant is designed to capture and cope with all wastewater treatment and recycling on-site. Consultant David Triggs explores the world of BedZedBuilt for the social housing landlords Peabody Trust, London's largest housing association the Beddington Zero Energy Development (BedZed) is a pioneering eco-housing project. The innovative design minimises negative environmental impact and provides homes for currently around 200 people in 80 dwellings ranging from single-bedroom apartments to four-bedroom townhouses and some 1,600m2 of flexible work space, including a number of work/live units.
The development incorporates both rainwater harvesting and wastewater treatment and recycling. The greenwater treatment plant is designed to capture and treat all the wastewater generated on-site to a level such that it can be re-used both for flushing the toilets within each dwelling and to provide irrigation water for the communal and private roof gardens. It is named greenwater since it is dyed a very pale shade of green as part of the safety strategy to ensure against any inadvertent use for other purposes. The service plumbing carrying the greenwater from the treatment plant to and within the apartments is also coloured green and uses different fittings for similar reasons. The development incorporates a range of water-saving fittings and appliances and the net effect of these, together with the water recycling, has been to reduce the household consumption of mains water by around 50% (to around 75 l/head/d) compared with current norms for the area. By fully treating all wastewater on-site, the discharge of sewage to the public system has been eliminated and clean effluent is made available for other on-site amenity, decorative or commercial uses.
A conventional drainage system discharges to a pumping station where flow is lifted in order to pass through a pair of large septic tanks in series. Effluent from the septic tanks is then pumped to the treatment plant, which is located within the CHP (social or sports clubhouse) building at the north end of the site. In addition to their normal function of providing gross solids separation and digestion, the septic tanks are configured to be hydraulically integral with the pumping station wet well so as to provide a flow balancing facility. This enables flow to be delivered to the treatment plant at a virtually continuous rate (for example, the pump operates for a set run time during a short pumping cycle, currently every four minutes).
In the greenhouse, flow is received into a small (189.25 litres) header tank, which further balances the flow and from where it splits to gravitate to a pair of smaller tanks serving the two treatment streams. The original design brief for the plant included the requirement that it would be housed within a greenhouse, that it should incorporate plants growing on the water surface and that it should look attractive and be suitable as an educational or visitor feature. Effluent needed to satisfy the Environment Agency's (EA) consent of 10mg/l BOD, 10mg/l suspended solids and 5mg/l ammonia and that portion be recycled into peoples' houses need to be disinfected such that the coliform count did not exceed 10cfu/100ml.
A key consideration in evaluating prospective designs for the plant was that of operability. This was of critical importance since it is the operator who would become responsible for delivering effluent on a day-to-day basis and that this would be carried out under the most transparent of circumstances (for example, the residents at BedZed would see the product every day in their homes). The requirement to operate an unconventional - and under the specific effluent requirements of the site, an unproven system - led to the need for a somewhat cautious approach. In particular, it was felt there was a need to incorporate a considerable degree of operational flexibility so the treatment process could be refined in the light of actual circumstances encountered.
Key to this operational flexibility, both for maintenance purposes and as part of treatment refinement, was the need not to be restricted to a single treatment stream and the ability to by pass and modify individual process elements. Given the greenhouse requirement, the living machine approach to sewage treatment was clearly attractive but an early proposal, based upon a redundant living machine that was to be relocated from a site in Sardinia, which was rejected on both the single stream and inflexibility criteria. In evaluating the living machine system, it seemed to this writer that it was essentially an extended aeration, activated, sludge plant operated at low-mixed liquor solids followed by a form of submerged aerated filter (styled an ecological fluidised bed). The extent of the contribution to treatment provided by the roots of the plants was not clear and while clearly real, providing a large surface area for attached growth, some oxygenation and nutrient uptake, did not seem to be very significant.
In comparing it with conventional activated sludge, it seemed to require rather a greater process volume and more treatment units.
It was therefore judged that provided the dual stream and inter-tank flexibility was achieved, an operator ought to be able to live with a living machine-type system, confident the plant could be adapted and operated as a conventional activated sludge plant if required. A major constraint was of course cost and a critical challenge was how to provide a sufficient number of tanks and overall tank capacity that was consistent with the living machine-type approach, coupled with the required operational flexibility.
The normal sources of cheap, recycled storage and process tanks were explored but nothing suitable was identified. Consideration then turned to the DIY option and a simple design was devised based upon using the same timber decking material that was to be a feature of the greenhouse itself. Each tank is in the form of a vertical cylinder barrel, 2.4m high x 1.5m diameter and has a capacity of around 4,000 litres. The tanks are sealed with a tailored butyl rubber liner over a layer of geotextile underfelt.
The top portion of the butyl liner is protected from abrasion by a strip of flexible plastic sheet and the top of each tank is capped by an octagonal ring of decking, which further protects and secures the collar of the butyl liner. The connection between tanks is via 50mm flexible hose with screwed unions and two lengths of similar hose is provided within the tank.
This enables the location of the feed and exit to each tank to be varied and, by raising the hose above the water level, the tank may be isolated. Air is provided by a variable speed side channel blower supplied by Becker UK and this is used both for aeration and to operate air lifts for the return and surplus sludge lines. The need to control odour was a serious concern, especially since the feed to the greenhouse was from a septic tank. The header tank and the two splitter tanks controlling the feed to each stream are therefore closed and the first aeration tank is also sealed.
Off-gasses from this tank may thus be obliged to pass through a biofilter consisting of a 600mm layer of compost or soil on which there is a small herb garden.This first tank also contains an anoxic zone and a returned sludge conditioning section, both of which have complicated the feed arrangement and do not appear to confer any significant benefit.
The plant was commissioned using fresh activated sludge from a nearby Thames Water plant and an initial problem was experienced to maintain an adequate level of mixed liquor solids. The plant rafts consist of a square tubular frame over which a net is stretched to contain a mass of coir fibre which support the plant stems. It was found that sludge solids tended to accumulate on the surfaces of the raft and within the coir fibre.These accumulations became somewhat unsightly and smelly and effluent quality was inadequate. Problems were also being experienced with pipe blockages as portions of the coir were carried into the outlet pipes.
The flexible pipe system described above enabled these to be cleared fairly easily but the dangers of overflowing were serious and it was decided to remove the plant rafts from the B stream.
This enabled higher MLSS to be adopted and performance quickly improved. It also enabled two of the aeration tanks on stream B to be bypassed even though it now received the bulk of the flow.
Excellent effluent has continued to be produced from the B stream and ammonia levels have consistently been less than 1mg/l. This has, however, been accompanied by classic rising sludge (denitrification) problems which at the time of writing still need to satisfactorily dealt with.
The final submerged aerated filter has however provided good effluent polishing and has prevented the rising sludge issue from affecting final effluent quality. The surface plants have been retained on Stream A and it now performs well although with full nitrification rising sludge can be a problem here also and it uses more tank capacity than stream B.
Refinement continues and the challenge now is reduce operating input by avoiding the denitrification problems in the clarification tanks. The most likely route for this would seem to involve the use of a membrane filter or similar to avoid the need for any final settlement. This would of course further reduce total tank volume requirements but still enable visually attractive polishing options to be employed if required