Planning For Drainage Needs In Christchurch

Press, Volume XC, Issue 27407, 21 July 1954, Page 9

Planning For Drainage Needs In Christchurch

Adequate planning for Christchurch sewerage would be on the assumption of a population of 500,000 at the end of this century, Professor C. G. Hyde, one of the two American drainage experts, indicated to the Christchurch Drainage Board.

The minutes of the meeting of the board last week with Professor Hyde and Dr. D. Caldwell, another American expert, who visited Christchurch for four days from July 9. were made available at last evening’s meeting of the board. The minute said: “Professor Hyde reviewed their work since their arrival in Christchurch on the previous Friday evening, during which period they had inspected various sections of the board’s activities, including the construction of relief sewers and new main pumping station, as well as thoroughly inspecting the sewage farm, where‘'they were impressed with the experimental work in connexion with the sewage treatment pilot plant. “A visit to the Port Hills had enabled them to obtain a good idea of the geography of the area controlled by the board.

“Professor Hyde expressed appreciation of the completeness of the data supplied by the staff, which, added to that already in their possession as a result of the visit of the secretary and engineer to Auckland, had enabled them to form the favourable opinion that the board and staff were planning their work soundly and With an eye to the future. In this connexion, he indicated that adequate planning would be on the assumption of 500,000 population by the end of the century, with an average population density of 10 to the acre, and an average quantity of 50 gallons per capita. “He expressed confidence in the staff to design sewers and pumpjng stations; and commended the gdod engineering and civic planning of the latter. “Dealing with the sewage farm, he stressed that this point of concentration was well located and could not be better. He outlined generally the proposals they would be putting forward for the treatment of the increasing quantities of sewage that would be pumped to the farm, and then asked Dr. Caldwell to elaborate. Treatment Plans Dr. Caldwell emphasised that the original method of treatment for which the farm had been designed (sand filtration) was highly efficient within its capacity but was now heavily over-

loaded. Another method of treatment was now necessary if the standard of effluent discharging into the estuary were not to deteriorate further. A changeover in three stages would be advocated, with an ultimate capacity of 33-35,000,000 gallons a day. The first stage would need to be completed in one or two years and he was happy to state that the exising tanks would fit admirably into the scheme. Full details of the type of treatment would be incorporated in their written report. Briefly the treatment would provide for:—The removal of suspended solids, stabilisation of the liquid effluent, removal or sterilisation of toxic bacteria.

The final scheme as proposed would comprise, set out in order of flow through the plant, the following:—Bar screens to remove rags, etc.; parshall flume to measure the flow; pre-aera-tion and grit removal tanks to improve the sewage for sedimentation purposes. (This is to be carried out on the first third of the length of part of -the existing septic tanks.); primary sedimentation in the remaining two-thirds of part of the existing septic tanks to remove suspended solids; sludge digestion tanks to be constructed adjacent to the existing septic tanks and sludge lagoons to hold the digested sludge, sited between the existing tanks and Pages road; high rate trickling filters to reduce the bulk of the pollution load in the effluent from the primary tanks; secondary sedimentation in part of the existing septic tanks to remove the suspended solids from the filter effluent; oxidation ponds of some 400 acres minimum area to remove part of the pollution load and to sterilse the effluent before discharge to the estuary.

These ponds are to be formed from the existing paddocks and approximately 3ft deep water depth. In addition as large an area as practicable of.the board’s property fronting on to the Estuary would be converted to

ponds; administrative building, with necessary offices, laboratory staff room, etc., sited near the existing tanks. Construction Stages Three stages of construction were recommended. One septic tank should be converted as soon as possible to re-aeration and primary sedimentation tanks, and the oxidation ponds screens and flume constructed.

After the population reached 200,000 in 1970, Dr. Caldwell recommended, some high-rate trickling filters should be constructed and another septic tank should be converted to partly primary and partly secondary sedimentation tanks to handle the increased load. More digestion tanks should be constructed. The third stage was to be •completed by the year 2000, or when the population reached 500,000. This involved constructing more high-rate filters and the conversion of one-third of the existing septic tanks to more pre-aeration, primary and secondary sedimentation tanks. More digesters should 4 be constructed as. required. The oxidation ponds at that stage would function entirely as sterilising agents. The design of the plant. Professor Hyde emphasised, should be done locally. But benefit would result from consultation from time to . tirhe. The chairman (Mr E. H. S. Hamilton) said that extracts from the letters of Mr D. L. Stevens, who was in charge of the board’s sewage disposal and was now studying at the Institute of Technology, Massachusetts, made it apparent that benefit could be derived from his studies. It was fortunate that the board had an engineer available to study the new works near Los Angeles and San Francisco.