Sewage is a mixture of domestic and industrial wastes. It is more than 99% water, but the remainder contains some ions, suspended solids and harmful bacteria that must be removed before the water is released into the sea. The treatment of wastewater is divided into three phases: pretreatment, primary treatment and secondary treatment. Pretreatment Large solids (i.e. those with a diameter of more than 2cm) and grit (heavy solids) are removed by screening. These are disposed of in landfills. Primary treatment The water is left to stand so that solids can sink to the bottom and oil and grease can rise to the surface. The solids are scraped off the bottom and the scum is washed off with water jets. These two substances are combined to form sludge. Secondary treatment The sludge is further treated in 'sludge digesters': large heated tanks in which its chemical decomposition is catalysed by microorganisms. The sludge is largely converted to 'biogas', a mixture of CH4 and CO2, which is used to generate electricity for the plant. The liquid is treated by bacteria which break down the organic matter remaining in solution. It is then sent to oxidation ponds where heterotrophic bacteria continue the breakdown of the organics and solar UV light destroys the harmful bacteria. The role of the laboratory A wide variety of analytical tests are used to determine the purity of the wastewater at various stages of treatment so that the possibility of harm to either people or the environment is minimised. INTRODUCTION Sewage is a major carrier of disease (from human wastes) and toxins (from industrial wastes). The safe treatment of sewage is thus crucial to the health of any community. This article focuses on the complex physical and biological treatments used to render sewage both biologically and chemically harmless. The Auckland region has two sewage treatment plants: one in Albany and one in Mangere. The process described below is that used by the Mangere treatment plant, which was built in 1960 and currently serves Auckland, Manukau and Waitakere Cities and the Papakura District. It is the largest such treatment plant in New Zealand, but its methods are similar to those used throughout the country. The waste treated is a mixture of domestic and industrial waste, with the domestic accounting for
slightly more than half of the total. Some stormwater also enters the system through leaks and illegal connections1. Volume and composition On average, 280 000 m3 of sewage arrives each day, although during winter storms this can swell to 800 000 m3. Of this, 99.9% is water. The remainder is mostly organic matter (800 - 1000 g m3 ) which constitutes the bulk of the suspended solids (250 - 400 g m-3). The biological processes which break down this organic matter require oxygen, and the amount of oxygen required is calculated as the wastewater's "biochemical oxygen demand" (BOD). Sewage coming into the plant (influent) has a BOD of between 200 and 400 g m-3 (i.e. 200 to 400 grams of oxygen are required to oxidise each cubic metre of influent). The remaining organic matter consists of the fat and grease that form a scum on the surface of the influent. As well as organic matter, small amounts of inorganic ions are also found. The most significant of these are chloride (100 - 200 g m-3) and sulphide (0.1 - 0.7 g m-3). Sulphide, despite its low concentration, is of greater concern than chloride because it is very foul-smelling even at this level. The influent generally contains no dissolved oxygen, so this must be added at various stages of the process to enable the organics to be broken down. THE TREATMENT PROCESS The purification works at Mangere provide both primary and secondary treatment processes. Primary treatment removes most of the solids from the effluent, but doesn't remove or degrade the dissolved organic matter. Secondary treatment uses microorganisms to convert these organics to simple...
Please join StudyMode to read the full document