SEWAGE TREATMENT

New Zealand Herald, Volume LXXII, Issue 22202, 31 August 1935, Page 16

SEWAGE TREATMENT

CROWN'S ISLAND SCHEME PLAIN SEDIMENTATION OUTLINE OF THE PROCESSES The methods ho has recommended to tho Auckland and Suburban Drainage Board for tho treatment of the sewage of tho southern end of tho proposed metropolitan drainage district, to result in the discharge of a clear ellliient into tho Motukorea Channel, wore described yesterday by Mr. H. H. Catkins, engineer and secretary to tho board. It is proposed to construct tho necessary treatment works at Brown's Island, over which the board has acquired an option to purchase. The system he advocated for use in connection with tho scheme was known as plain sedimentation, Mr. Wat-kins said. The sewage would flow by gravitation to Urown's Island, where it would be pumped to a sufficient height to flow through the treatment plant. This would consist of coarse screens, detritus-removal chambers, greaseskimming chambers and tho sedimentation tanks. Meeting Future Requirements The capacity of the tanks was designed to ensure tho slow mo\ cmont of the sewage, thus permitting the settleable solids being precipitated. On top of these solids, there would be left a clarified liquid, which would then flow over weirs and be chlorinated before being finally discharged into tho Motukorea Channel through tho main outfall of the sower.

"The thick liquid deposit produced in tho course of the treatment of tho sewage is commonly known as sludge, Mr. Watkins continued. "This sludge is slowly scraped by mechanical equipment along tho bottom of tho tanks to the centre, and is then removed bv hydrostatic pressure. The sludge to bo produced on Brown's Island could be satisfactorily dealt with on drying bods for several years, but, as the size of tho plant is increased to provide for greater areas, it is intended to ship tho wholo of tho wot sludge to sea, similarly to tho method in operation for tho London, Manchester and Glasgow main drainago districts." Construction of Tanks In his report to the board Mr. Watkins said that by tho sedimentation tank process it was possible* to remove from 50 to 75 per cent of the suspended matter from sewage of an average strength, being equivalent to a volume of from five to ten times tho amount of suspended matter removable by fine screening. It was worthy of note that the tanks could bo designed and constructed so as to bo adaptable for treatment of the non-settling organic impurities in the sewage, if at any future period it should bo desired to adopt such a process. While abroad some years ago he found that there had been a marked improvement in the general status of the plain sedimentation method of sewage disposal during the last decade, Mr. Watkins said. Tho process was particularly adaptable to the average requirements of drainage districts from which sewage was discharged into tidal waters.