A case of long-term treatment

Like WWT, Maple Lodge STW in Hertfordshire is 50 this year. Looking back at the site's history Malcolm Hallsworth finds a microcosm of the UK's wastewater treatment industry

In 1933 the clerk of Rickmansworth council in Hertfordshire invited proposals to deal with the town’s sewage disposal problems. Seventeen years later his efforts finally bore fruit with the commissioning of Maple Lodge STW in 1950. The works has been in continuous operation since.

In the 1930s, west Hertfordshire’s sewage was handled by numerous small filter works and even larger numbers of septic tanks. In a scenario that was far from unique, the works were ageing and seriously overloaded. At the time as Rickmansworth council sought a local solution to the problem, the then Ministry of Health was addressing the matter at a regional level, looking at the drainage of greater London.

By the middle of the decade the two bodies combined efforts. It was planned to replace most of the existing sewage disposal works and pumping stations with trunk sewers and a centralised STW. As one of 10 schemes recommended by the Ministry of Health in its Greater London Drainage Report, the new network was to serve the watershed of the River Colne and its tributaries the Chess, Gade and Ver. A parliamentary bill was drafted to enable the setting up of a joint sewerage board and in 1937 the Colne Valley Sewerage Board came into being.

Once the design for the new treatment regime had been finalised, Messrs F Smith and Son of Grimsby were contracted for £210 18s to sink trial boreholes. Work progressed apace, contracts worth £500,000 were agreed then World War Two broke out. Construction was wound down and finally ceased in December 1940. For the next five years only vital maintenance work was undertaken and moorhens took up residence in the half-built, flooded power house.

Among the workforce when the project resumed in 1945 were 500 German prisoners of war, used to ease the shortage of labour. Another problem was that many of the partially-completed contracts were resumed by different companies. But the most serious obstacle was the announcement of new towns within the catchment area and new restrictions upon the development of areas through which the mains sewers had been planned Ð much of the scheme had to be redrawn. Finally, by the end of 1950 testing was complete and Maple Lodge was deemed ready to receive its first sewage flows the following year.

The site was seen as a triumph of mechanisation with many innovative features. New technology meant a single 26-acre site replaced local disposal works with an aggregate footprint of 500 acres.

Apart from the updating of plant and control systems, the treatment process initially employed at Maple Lodge remains essentially unchanged. The works is a fully nitrifying diffused-air activated sludge plant with heated anaerobic sludge digestion.

Primary treatment was through four grit channels, each capable of passing 22MG/d. After the grit channels sewage passed through storm weirs, a measuring flume and into the comminutors. The automatic control of grit channels and comminutors was seen as very advanced. As flow increased additional channels or comminutors were brought into action, motorised penstocks and the comminutors’ motors were governed by the sewage levels in the measuring flume. Metering equipment was supplied by George Kent, the comminutors by Jones and Attwood.

Sedimentation took place in 900,000G tanks, sewage then entered one of eight 750,000G aeration lanes. Average aeration time was about 12 hours. Final settlement was undertaken in 12, 250,000G tanks.

According to a Colne Valley Sewerage Board booklet of the day: “The methods used for dewatering sludge and also the surplus activated sludge, together with the method of sludge drying, constitute a feature of special interest at the Maple Lodge works. The plant involved is probably the largest and most complete installation outside the United States.”

Drying out

The dewatering system used two elutriation tanks and chemical conditioning with a mixture of ferric chloride and ferric sulphate produced by chlorination of a solution of ferrous sulphate. Wallace & Tiernan devised the chlorination plant. The drying plant comprised six vacuum filters each with a surface area of 300ft2 – woollen cloth was used as the filtration membrane. Flash drying completed the process.

Over the subsequent 50 years population pressure, improved technology, the need to cut costs and environmental protection legislation have all played a part in shaping Maple Lodge. The first major extension took place in 1966, as a result of increasing flows. The work comprised four new sludge digesters, a bank of new aeration units, extra blower capacity, extra sedimentation tanks and new inlet works.

As the 1960s progressed it became apparent sewer capacities were becoming inadequate and it was undesirable for ever-increasing sewage effluent discharges to be released into the River Colne at a single point. Upstream of Maple Lodge, the river’s flow was very low, causing virtual stagnation for lengthy periods of the year. To cope with the problem a smaller activated sludge relief works, Blackbirds Farm, was opened. The new STW drew a set aliquot of sewage from the main trunk sewer feeding Maple Lodge, and treated around one sixth of Maple Lodge’s dry weather flow. Work began in 1973, Blackbirds Farm discharges into the Colne about 12 miles upstream from Maple Lodge.

The 1990s witnessed two major improvement exercises. Automation of the sludge handling process, primary desludging, digester feeding and improved digester heating led to a substantial increase in Maple Lodge’s sludge treatment capacity. It was now possible for the site to treat sludge from small surrounding STWs. Perhaps of greater significance, and a sign of advances in electronic process control and monitoring systems, was the introduction of a supervisory control and data (SCADA) system. For the first time operation of all major plant processes and equipment, at both Maple Lodge and Blackbirds Farm, could be conducted from a central control room, by just one person.

Staffing levels represent an area of significant change at Maple Lodge. Colne Valley Sewerage Board documents refer to: “A small estate of twenty houses in Maple Cross [built for] the accommodation of employees, principally key maintenance men and senior shift workers.” Originally the works was a labour-intensive, self-contained site with its own clerical and administration staff – even the wages were processed onsite – there were also process, electrical and mechanical engineering teams and laboratory staff. The process team alone had three onsite managers.

Today such prodigious levels of manning are unheard of. A potent mix of IT, commercial considerations and changing business structures mean Maple Lodge and Blackbirds Farm operate under the supervision of a single shift worker, while laboratory, engineering and administration support are all centralised within Thames.

The latest work, which brings the story pretty much up to date, began in 1997 and finished in October 1999. In order to meet ever more stringent discharge consents, Thames Water sought additional effluent treatment capacity in addition to improvements in the existing process plant.

As principal contractor, Balfour Beatty undertook the civil work, Brown & Root was the designer. In a far cry from the £500,000 spent during the first two years of Maple Lodge’s construction, £16.9M was spent on the latest upgrade. In another sign of the times, a teamwork approach was adopted, the contract was carried out following the tenets of a team charter, and early in proceedings members of the client, construction and design teams embarked on a residential team-building course.

A substantial programme of works included new screenings, conditioning plant, new primary and surplus activated sludge processing plant and refurbishment of primary and storm tanks. With computer control playing an ever-more significant role in virtually every area of the works the existing instrumentation control automation (ICA) provision at both Maple Lodge and Blackbirds Farm was upgraded. New programmable logic controllers (PLCs) were provided for new plant, those already in-situ were upgraded. The all-seeing SCADA system was also revamped.

The work also ensured that the site which once boasted the largest, most complete sludge dewatering and drying facility outside the United States now had the largest tertiary treatment plant of its kind in the UK. The plant, installed by Degremont, comprises eight sand/membrane filter beds with automated backwash via moving bridges. Capacity is 300Ml/d.

As long as the population of the southeast continues to swell and discharge consents carry on getting stricter, Maple Lodge will continue to improve its standards and capacity of sewage treatment and sludge handling. Although as those who commenced the building work in 1938 found, the future is never certain

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