Plant Already Needs Extension

Press, Volume CVI, Issue 31088, 17 June 1966, Page 29 (Supplement)

Pl an t Already Needs Extension

Planned to have additions in 1975 to cope with a population of half a million, the sewage treatment works at Bromley are already coping with loads which only a few years ago were forecast for nine years hence and now the Christchurch Drainage Board is planning extensions to meet the needs of a population of 700,000.

The modern plant opened late in 1962 in spacious grounds at Bromely, replaced the old sewage farm, which in its day was a progressive step in disposing of the city’s sewage. Earlier, sewage was discharged into primitive settling basins in the sand dunes, ultimate disposal being by percolation through the sand. When the farm was established sewage was pumped to it and passed through three large open septic tanks. The effluent was spread through a system of open channels over the 225 acres of land formed by levelling the original sand dunes. The “filtered” effluent then went by deep drains into the Estuary. This method of treatment, although it provided an effluent of a high standard, had limitations, and in 1954 the board engaged two engineers from San Fransico to report on a programme for the development of modem disposal methods. This was to begin with the construction of a plant capable of serving a population of 300,000 and which could ultimately cope with a population of 500,000. The American consulting engineers were engaged to translate the programme into detailed design drawings, and an engineer of the board was sent to America to work on the project The project was financed bv loans amounting to £1.33 m.

Sewage from all parts of the metropolitan area is pumped to Bromley, where it flows to a screen room where coarse solids are removed from the flow, and

then goes to tanks for grit removal and preaeration. From there the sewage passes through sedimentation tanks, and then, virtually free of solid matter, is pumped to the trickling filters. Distributed over a bed of stones, the sewage percolates down before going to secondary sedimentation tanks for final clarification; • Finally the effluent goes to the oxidation ponds, which cover 550 acres. The ponds provide a natural sanctuary and are a wildlife refuge, providing a haven for bird life which has become established since the ponds were filled. The plant makes use of one of its own by-products —sludge gas produced by a breakdown of volatile organic matter. The gas is used as a fuel for the engines driving the main pumps and blowers.

Manual operation at the plant has been reduced to a minimum by the use of electronic control equipment. By incorporating the many control devices in a central panel, supervision of the many processes can be made without the necessity of frequent inspections by the staff. Levels, pressures, temperatures and rates of flow are measured and recorded where necessary. Sensitive pneumatic and electronic control instruments sense variations from the normal control settings, and values, motors and pumps react to the pneumatic and electrical signals transmitted by the instruments.

Associated with the plant is a laboratory to provide the information on which to control operation of the works.