Toxicity management: it's a bug's life

Toxicity control is a major driving force in the run up to the Urban Waste Water Treatment Directive. Dr Peter Spencer Davies, director of Strathkelvin Instruments, explains what needs to be done and how.

This aeration tank has been affected by toxicity and is working at sub-optimal capacity

This aeration tank has been affected by toxicity and is working at sub-optimal capacity

Most biological treatment plants that accept industrial wastewaters experience problems from time to time with nitrification. This can result in failure of the treated effluent to meet ammonia discharge consent levels, and can lead to substantial fines.

Toxicity also affects the carbonaceous bacteria and results in a reduction of their respiration rate, in rate of growth of biomass and in the rate of breakdown of the Biological Oxygen Demand (BOD). In extreme cases, toxic shock can close the plant completely.

When this happens the costs of sludge removal and reseeding can be very high. Many companies are now realising that it makes economic sense to adopt a toxicity management plan in order to avoid these extra costs – and the effect it has on the company’s reputation.

Toxicity management
A toxicity management plan has four parts. Firstly, the toxicity of all waste streams entering the main influent stream needs to be screened. This is done by collecting samples at source and measuring the toxicity to both the carbonaceous and nitrifying bacteria of the receiving activated sludge.

Secondly, if significant toxicity is detected, steps should be taken to remedy this before discharge. The effectiveness of the toxicity reduction procedures can be quickly tested in the laboratory.

When control of the toxicity of discharges has been achieved, the next stage is to protect the plant from toxicity entering the aeration tanks. This requires on-line toxicity monitoring. And, finally, the effectiveness of these procedures needs to be checked by taking daily samples of the activated sludge and carrying out rapid tests of its ‘health’, by measuring respiration and nitrification rates.

Testing for toxicity
Until now there has not been a fast, reliable test to measure the toxicity of industrial wastewaters to the nitrifying bacteria. To meet this need, Strathkelvin Instruments has just launched a new nitrification inhibition test using its Activated Sludge Respirometer (ASR).

The test is based on the principle that the total oxygen uptake of a nitrifying sludge comprises the oxygen taken up in nitrifying ammonia to nitrate, and the respiration of the carbonaceous bacteria.

Samples of sludge are mixed with five concentrations of the test wastewater, and the total oxygen uptake measured. A second run is then made, after adding the nitrification inhibitor, allyl thyourea. The computer software subtracts the carbonaceous respiration of the second run from the total uptake of the first, to calculate the nitrification rates and the nitrification inhibition.

The test can be carried out by semi-skilled personnel in under half an hour. Since it uses the actual nitrifying bacteria in the activated sludge of the receiving plant, there are no expensive consumables to purchase.

However, the big advantage, is that it reports on both nitrification inhibition and inhibition of the respiration of the carbonaceous bacteria, within the one test. All of the difficulties associated with respirometry, have been overcome by the easy-to-use software, and the end result is the automatic production of a fully audited toxicity report that can be printed out.

The updated software for the respirometer, also includes tests for short-term BOD, and an activated sludge ‘health’ monitoring program in order to provide an early warning of possible toxicity-based problems.

Avecia has been using the ASR in its effluent management programme at its Grangemouth site. Avecia was formed by a buyout of the speciality chemicals section of AstraZeneca in 1999. It now operates from 28 sites worldwide with Grangemouth being the main manufacturing site in the UK, specialising in fine chemical production such as ink jet printer dyes, additives, agrochemical and pharmaceutical intermediates and advanced therapeutics.

The aqueous wastewaters of the manufacturing process are treated on-site in a chemical and biological treatment plant with up to eight megalitres of treated effluent being discharged each day to the Firth of Forth. Because it produces a range of manufactured products, it is not surprising that its wastewaters may contain substances of varying potential effects on effluent plant biomass.

Optimal efficiency
Protection of the treatment process has been a long established policy at the site but a biological treatment plant operating below optimal efficiency because of the effects on the sludge bacteria cannot fulfil its design intent and also incurs costs to the company.

The responsibility for the monitoring of wastewater falls to the Environmental Group within the Analytical Laboratory. For many years, the Group used an Activated Sludge Respiration Inhibition Test (ASRIT) based on OECD 209 Guidelines. With the launch of the ASR, Avecia bought a unit which is now in continuous use, making considerable on-going cost savings in effluent testing. The unit actually paid for itself during the evaluation, training and establishment period.

Effluent testing forms an important part of product research and process development. Any new chemical, before being granted a production licence by the regulators, has to undergo statutory testing of its effects on biological systems. This is carried out by approved contract laboratories at the appropriate time but Avecia is able to pre-test all new raw materials, products, by-products and wastes and to work on any necessary modifications at early research and development stages.

Similarly, the wastes produced from existing processes are monitored in the laboratory, before discharge to the treatment system. If they are found to be more bio-inhibitory than the established standards, reduction procedures are worked up for pre-treatment of the waste before discharge.

In operation
Although the Activated Sludge Respirometer is a new technology, it is already in use by major water companies and by manufacturing companies that treat their own waste throughout the UK, Ireland and in northern Europe.

Strathkelvin Instruments also advises on the structure and operation of a toxicity management plan for any treatment works that takes in industrial wastewater, in order to ensure that its treated effluents remain compliant.


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