The biological basis of wastewater treatment

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Courtesy of Strathkelvin Instruments Ltd.

Introduction

This booklet was written to fulfil the need for a simple explanation of the biological processes that underpin wastewater treatment. It attempts to show how the bacteria involved deal with the organic carbon in the sewage. Remarkably, there are just 3 major processes involved, and these mirror exactly the 3 major processes at work in the plant viz: biodegradation, oxygen removal from the water, and the production of sludge. The article is divided into two parts. The first section deals with the biology of the bacteria. In the second section, the ways in which these processes underpin the management of a wastewater treatment plant are explained. Inevitably in a brief overview, such as this, much has had to be left out. However, it is to be hoped that this will be justified by the clarification and simplification of the underlying principles.

Biological treatment by activated sludge

Wastewater comes from two major sources: as human sewage and as process waste from manufacturing industries. In the UK, the total volume of wastewater from industry is about 7 times that of domestic sewage. If untreated, and discharged directly to the environment, the receiving waters would become polluted and water-borne diseases would be widely distributed. In the early years of the twentieth century the method of biological treatment was devised, and now forms the basis of wastewater treatment worldwide. It simply involves confining naturally occurring bacteria at very much higher concentrations in tanks. These bacteria, together with some protozoa and other microbes, are collectively referred to as activated sludge. The concept of treatment is very simple. The bacteria remove small organic carbon molecules by ‘eating’ them. As a result, the bacteria grow, and the wastewater is cleansed. The treated wastewater or effluent can then be discharged to receiving waters – normally a river or the sea.

Whilst the concept is very simple, the control of the treatment process is very complex, because of the large number of variables that can affect it. These include changes in the composition of the bacterial flora of the treatment tanks, and changes in the sewage passing into the plant. The influent can show variations in flow rate, in chemical composition and pH, and temperature. Many municipal plants also have to contend with surge flows of rainwater following storms. Those plants receiving industrial wastewater have to cope with recalcitrant chemicals that the bacteria can degrade only very slowly, and with toxic chemicals that inhibit the functioning of the activated sludge bacteria. High concentrations of toxic chemicals can produce a toxic shock that kills the bacteria. When this happens the plant may pass untreated effluent direct to the environment, until the dead bacteria have been removed from the tanks and new bacterial ‘seed’ introduced.

Globally, the composition of effluents discharged to receiving waters is regulated by the national environment agencies. In Europe the regulatory legislation is the Urban Waste Water Treatment Directive (1991) and the more recent Water Framework Directive (2000). In the USA, the Environmental Protection Agency (EPA) ensures compliance with the Clean Water Act (1977). The legislation is concerned with the prevention of pollution, and therefore sets concentration limits on dissolved organic carbon (as BOD or COD), nitrogen and phosphates – which cause eutrophication in receiving waters. It also attempts to limit the discharge of known toxic chemicals by setting allowable concentration limits in the effluent. Recently, in recognition that effluents contain unknown toxic chemicals, a more pragmatic approach to regulation is being introduced in Europe, using Direct Toxicity Assessment (DTA) tests. In the US these have been in use for many years and are known as Whole Effluent Toxicity (WET) tests. These tests are used to measure the toxic effects of effluents on representative organisms from the receiving waters. Any toxicity detected in the effluents will obviously have been present in the sewage entering the plant. Surprisingly, direct toxicity assessment of influents to wastewater treatment plants that could impact on the functioning of the bioprocesses is not yet included in legislation.

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