Protecting receiving waters
Peter Davies of Strathkelvin looks at toxicity management at WwTWsThe effective management of activated sludge from a biological treatment plant is of critical importance in order to minimise treatment costs and to avoid pollution of receiving waters by discharged effluents. Plants that treat industrial waste may find that toxic or inhibitory chemicals can pass through treatment systems with little effective removal. 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. 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.
In some cases, toxicity does produce observable indications of operational difficulties. Settling problems can be caused by both filamentous bulking and by deflocculation. Both of these are induced by toxic chemicals in the mixed liquor. 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.
A recent study on the Antwerp treatment works showed almost continuous inhibition of respiration rate of the activated sludge over a 10-day monitoring period. Peaks were up to 30%, with an average of 10% respiration inhibition. No deterioration of effluent quality was observed during this period. However during a second 10-day period, there was a 48-hour increase in toxicity with peaks of up to 43% and this was accompanied by a substantial washout of solids.
Toxic shock or the complete kill off of the activated sludge bacteria is not common and not normally publicised, but examples of this, and a recognition of the associated costs of sludge removal and reseeding, are well known within the wastewater industry.
Most legislation is directed towards regulation of discharges to the receiving environment, rather than to WwTWs. However, annex 1 of the EU Urban Waste Water Treatment Directive (1991) states: ‘Industrial wastewater entering collecting systems and urban waste water treatment plants shall be subject to such pre-treatment as is required in order to: ensure that the operation of the waste water treatment plant and the treatment of sludge are not impeded’. This has been largely overlooked and has not been the subject of secondary legislation, although in the UK some water companies do use toxicity-based discharge consents in some instances.
The more recent EU Water Framework Directive (2000) is concerned primarily with protecting receiving waters from pollution. The scope of regulation will increase beyond existing levels to place more emphasis on reduction of toxic chemicals entering receiving waters. Currently some discharge consents are based upon concentration limits of known toxic chemicals. However in recognition of the fact that a discharger often does not know the exact composition of the waste stream, and also that the toxicity of many of the discharged chemicals is not known, a more pragmatic approach will be used. Using a series of direct toxicity assessment (DTA) tests, the toxicity of end-of-pipe effluents to a series of prescribed organisms from marine or freshwaters, can be measured. Tests of this sort have undergone several years of field trials by the Environment Agency (EA), the Scottish Environmental Protection Agency (SEPA) and representatives of the chemical industry. Although the date and manner in which these will be used in formulating discharge consents is not yet clear, it is likely that future integrated pollution prevention and control (IPPC) consents will incorporate these DTA tests.
Managers of treatment plants that treat industrial wastewaters are now beginning to use a toxicity management plan to safeguard their works. The four elements of this are:
- regulation – toxicity of tankered waste is measured before acceptance for treatment. For discharges to sewer, the toxicity of all identifiable waste streams should be known,
- pre-treatment – manufacturers producing very toxic waste are required to pre-treat to effect toxicity reduction,
- effluent monitoring – an on-line toxicity monitor checks the toxicity of trade effluents arriving by sewer before discharge to the treatment tanks,
- monitoring of sludge ‘health’ – health is monitored daily by measuring the respiration rate and nitrification capacity of samples of activated sludge.
In order to meet the growing demand for easy-to-use and relevant toxicity tests, Strathkelvin Instruments launched a dedicated laboratory Activated Sludge Respirometer two years ago. This is now in use by water companies, companies accepting tankered wastes for treatment and chemical manufacturers that treat their own wastes. It is also used by process engineers at the critical stages of commissioning new plant when the unadapted sludge bacteria are most susceptible to kills by toxic effluents.
The Strathkelvin Activated Sludge Respirometer measures toxicity to both nitrifiers and carbonaceous bacteria in a single test, which takes less than 30mins. The tests are carried out on the actual activated sludge that will receive the industrial effluent. They therefore give the treatment plant manager information that is directly relevant to the performance of his plant.
The respirometer results from technology transfer from respirometry applications in the biomedical field. It uses six oxygen electrodes to measure the oxygen concentration in the tubes containing the respiring and/or nitrifying sludge. The software records and then analyses the rates of oxygen uptake before automatically producing a printable report. Four different software programs are provided, enabling the respirometer to be used additionally for process optimisation applications, such as short-term BOD measurement and tests that can be used for aeration requirement predictions.
In operation, the respiration rate of a control sample of sludge is compared with the rates from samples of the same sludge mixed with five different samples of the wastewater. Outputs from the oxygen electrodes are displayed as scrolling traces on the computer screen. Using sludge with a mixed-liquor suspended solids (MLSS) of approx 4,000mg/l, the respiration rates of the six samples are recorded in 5-10mins.
The analysed results of the test are displayed as a fully audited report in a locked spreadsheet. The report shows values for EC5, EC10, EC20 and EC50 i.e. the concentrations which produce a 5%, 10%, 20% or 50% inhibition of the respiration rate. These values can be used by the treatment plant manager to determine the rate at which the wastewater should be allowed to enter the plant, in order to minimise the effects of toxicity on the activated sludge.
Figure one shows the result of a test on tankered waste of unknown composition, which was delivered for treatment at a biological treatment plant.
It can be seen that with a 20% concentration of the wastewater, the respiration rate at 120.2mg O2/l/h is 3-4% below the respiration rate of the control sludge. With increasing concentration, the percentage inhibition progressively increases.
The software has calculated the concentration causing 5% inhibition (EC5) to be 21.1% while the EC10 is 24.9% and the EC20 is 34.8%. These values clearly show the waste is toxic. The plant manager then has to decide what level of inhibition is acceptable in the treatment tanks. In the study carried out at the Antwerp WwTW, an average 10% inhibition did not appear to affect the quality of the effluent. Most managers will probably opt for a more cautious approach and accept a 2-5% inhibition. Unfortunately there is still a dearth of information on the effects of low levels of toxicity on the treatment process.
The final stage involves the calculation of the dilution of the waste that would be achieved when discharged into the system, from knowledge of tank volumes and flows. From this it is then possible to calculate the rate of discharge required to achieve the acceptable level of inhibition