Wastewater treatment plants are coming under increasing pressure to increase their efficiency of operation. There are two objectives in this. The first is to reduce the cost of treatment. The second is a response to new legislation aimed at reducing the carryover of toxic materials to receiving waters. In order to achieve this, it is necessary to focus on the pivotal role of bacterial respiration in the aerobic treatment process. The importance of respiration in this context is that it is quickly and easily measurable. Using new technologically advanced respirometers it is possible to use respiration measurements for modelling the treatment process, for management of toxicity, and for measuring short term BOD, nitrification capacity, readily degradable BOD treatment capacity, and aeration requirements. Major increases in efficiency are most likely to come from more efficient process control and from the introduction of toxicity management plans for plants that treat industrial wastes.
Pressure to increase the efficiency of operation of biological wastewater treatment plants is coming from two sources. Firstly, there is increasing awareness that the economic costs of treatment can be reduced if the plant is operating at full efficiency. Computer modelling of treatment processes is making some progress, and this will lead to a better understanding of the underlying costs. Secondly, there is increasing pressure to improve the quality of effluents that are discharged, and in particular to minimise the carryover of toxic chemicals, which have not been removed in the course of treatment. Respirometry is an essential tool for both efficient process control and for managing the toxicity of those plants that treat toxic industrial wastes.
In the highly regulated environment in which the wastewater industry operates, changes in operating procedures often follow from changes in legislation. The Water Framework Directive (2000) seeks to improve the quality of receiving waters. In addition to regulating the discharge of BOD, COD, ammonia etc, which can directly influence eutrophication, toxic substances in effluents will be more tightly regulated. In particular IPPC licences will, in future, include provision for measuring the toxicity of the effluent to the actual organisms of the receiving waters. This will involve use of the new Direct Toxicity Assessment (DTA) tests that have recently been approved by the Environment Agency and SEPA. Clearly if toxicity entering a treatment works is not removed by the activated sludge bacteria, there will be failed discharge consents. The main reason for failure to remove toxicity, is that the bacteria themselves have been poisoned by the toxic chemicals, and are unable to function effectively. Legislation to regulate the discharge to public sewer of substances that will harm the functioning of a biological treatment works is contained in schedule 4 of the Urban Wastewater Treatment (England & Wales) Regulations, 1998. However, although effective procedures to measure influent toxicity and its effects upon the bacteria, are now available, the regulations are still not being effectively applied.
Exceptionally, treatment works operated by some of the larger manufacturing companies do regulate influent toxicity in order to maximise process control and minimise treatment costs. By so doing, they are also able to ensure that their discharged effluents are of high quality, and therefore to not attract the unwelcome publicity associated with failed consents.
Probably the most sensitive indicator of toxicity is inhibition of the respiration rate of the activated sludge. As will be shown below, when respiration is inhibited, the rate of biodegradation of the sewage is reduced. Respirometry therefore has an important role to play in detecting toxicity and in process control.