The increasing importance of assessing toxicity in determining sludge health and management policy

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The harm caused by toxic industrial effluents to sewage treatment works, has been recognised for many years. These can cause reduction in biodegradable efficiency, with an associated requirement for longer treatment time, and in extreme cases, toxic shock can kill off the secondary tanks.

Currently, the presence of harmful industrial effluents is made evident by their effects on the overall treatment cycle at the works. An increasing frequency of incidents has led to a call to establish or improve methods of detecting toxicity of effluents accepted for treatment.

This paper describes the causes and effects, discusses economic and operational penalties, and reviews the current approach to, and methods of detection. The paper ends with an appraisal of the likely advances over the next few years and the ways in which improved detection can lead to a toxicity management strategy for the operation of sewage treatment works.

Introduction

The effective management of the activated sludge of a biological treatment plant is of critical importance in order to minimise treatment costs and to avoid contamination by effluents of receiving waters. Even in well managed treatment systems, it is not uncommon for influent characteristics to change rapidly and unexpectedly as a result of changes in upstream discharges. Plants that treat industrial waste may find that toxic or inhibitory chemicals can pass through treatment systems with little effective removal (1,2,3). Storm flows can introduce toxins from leachates and other urban runoffs. The reduction of treatment plant efficiency resulting from the effects of toxicity on the activated sludge bacteria can therefore result in unacceptable levels of effluent toxicity.

Toxicity can also give rise to operational problems such as changes to the sludge settling characteristics, and this may take a significant time to recover. Settling problems can be caused by both filamentous bulking and by deflocculation. Both of these can be induced by toxic shock loads (4). Filamentous bulking may be detected by microscopic examination, enabling corrective action to be taken. However, the only indication of toxicity-induced deflocculation may be a rapid increase in effluent suspended solids. This may result in significant activated sludge washout before the cause and source of the problem have been identified.

The relationship between the observed consequences of toxicity shocks of this sort and the way in which the toxins affect the metabolic processes of the sludge bacteria is not well understood. This is unfortunate, since toxicity detection methods inevitably have to be based upon knowledge of this sort. The bacteria break down organic compounds in the mixed liquor, thereby reducing BOD and COD. The carbon removed is used either for the respiration of the bacteria or is incorporated into new biomass, as a result of growth. Any stage in the chain of metabolic reactions involved in both respiration and growth, can be poisoned by different toxic chemicals. As the respiration rate and growth rate is inhibited, the rate of breakdown of the carbon compounds of the mixed liquor decreases. As a result, the rate of BOD removal decreases. So toxic shocks can result in increases in effluent BOD and COD levels unless remedial action is taken. Similar effects on the nitrifying bacteria result in increases in ammonia levels in the effluent. In extreme cases the bacteria are killed by the toxicity. The restoration of a plant which has experienced total killoff, involving cleanout and reseeding, is a costly operation.

The role of toxicity management in treatment works is now receiving more attention from plant managers, largely as a result of the introduction of recent environmental legislation. However, there are also good economic and operational reasons for doing this. In this paper procedures and methods which are becoming available for the management of toxicity will be reviewed.

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