32 Water/Wastewater


the individual substances to be studied, or their breakdown products, or sometimes nitrogen or phosphorus compounds.


For comparison, alongside the actual biodegradability test there is always a blind test in which only the artificial wastewater, that is to say without the test substance added, is handled in the laboratory wastewater treatment plant.


The following are studied:


• Does the degree of degradation change due to the substance added to the water (measured, for example, as COD or TOC of discharge versus influx) compared to the artificial water to which nothing is added? This gives an indication of the possible toxic or inhibitory effects of this substance.


• Does the concentration of the substance to be studied change as it passes through the laboratory treatment plant - is it broken down or bound in the sludge - or does it run through unchanged? This yields information about the biodegradability of this substance.


• Is the substance easily biodegradable from the first moment, or must the bacteria first become accustomed to it?


• Does the biodegradability depend on the age of the sludge, that is to say the pollution of the treatment facility? In a highly polluted sedimentation tank, after all, fast-growing bacteria that only utilise the substances that degrade most easily quickly dominate; only with minimal pollution do slower-growing bacteria also have a chance.


Laboratory wastewater treatment plants


Laboratory wastewater treatment plants have the task in these biodegradability tests of mapping the processes in the treatment plant on a small scale. Much like a large treatment plant, they have aerated activation tanks, sedimentation basins and sludge return


pumps. They are available in various configurations, from simple single-stage treatment plants to plants with a denitrification phase.


The picture shows a plant with an activation phase and a denitrification phase. From a storage tank, a pump transports the wastewater into the activation tank. It is filled with activated sludge, usually that of the nearest treatment plant. It is aerated with a controlled air pump, and stirred with a stirring motor. The discharge of the activation tank ends up in the denitrification tank. There it is also stirred, but not aerated, so that the bacteria must use the nitrate in water as a source of oxygen. Thus the content during the course of ‘fertilizing’ (eutrophicating) nitrogen compounds is reduced.


From the denitrification tank the water enters the settling tank. There the sludge settles at the bottom and is pumped back into the activation tank; the treated wastewater flows into the collection tank.


Example: Degradation of bisphenol A in biological wastewater treatment


An illustrative example of the use of laboratory treatment plants is provided by the tests on the degradation of bisphenol that were carried out by M. GEHRING et al at the TU Dresden.1


Bisphenol A is used, for example, in the production of plastics, component adhesives and plasticisers. Although the compound was originally developed precisely because of its hormone-like action, it was only determined later on that it can develop adverse effects in the aquatic environment.


In the test series at the TU Dresden, a synthetic wastewater with bisphenol A concentrations of some 5 to 15 µg/l was collected and treated in a laboratory wastewater treatment plant. The flow rate was 0.5 l/h, that is to say 12 litres per day. The experiment was continued for 8 weeks; the bisphenol-A content was analysed weekly at various points in the treatment process:


• in the intake • in the water in the activation tank • in the water in the denitrification tank • in the discharge • in the sludge in the activation tank • in the sludge in the denitrification tank


This yielded a quite differentiated picture of the situation during biological degradation:


• Bisphenol A was found to be biodegradable, but the microorganism flora took several weeks to adjust to this wastewater component.


• To a considerable extent the substance is adsorbed into sewage sludge; in response to an incorrect transport procedure (if the sludge becomes anaerobic in the settling tank) it can, however, be released back into the discharge water from here.


• With a proper procedure it was possible to suppress the concentration of bisphenol A in the treated water under the detection limit.


Course of bisphenol-A concentration in water and sludge at various points in the treatment process (redrawn after1


) 1 M. Gehring et al., Vortragsskript, TU Dresden 2002, http://rcswww.urz.tudresden.de/~gehring/deutsch/dt/vortr/020419ge.pdf


What are the benefits of the study in a laboratory treatment plant?


The biodegradability test in a laboratory treatment plant makes it possible to study such processes on a small scale under conditions that are close to reality:


• Continuous processing of the test substance in realistic concentrations over any period of time makes it possible to observe the adaptation of the activated sludge to the test substance.


• Sludge return allows the formation of sludge with a sludge age that is near reality.


• The opportunity to take samples of the water and the sludge at different points in the treatment process allows a differentiated investigation of the processes involved in the biological treatment process.


Biodegradability conditions as these were demonstrated in the quoted test occur in many substances. The complicated interplay of adsorption on sludge, re-release and biodegradation or slow acclimatisation of activated sludge to a wastewater component could, for example, not be studied in a BOD measurement.


New Attitudes and Designs for Respirometers


Respirometers from Respirometer Systems and Applications (USA) are different than other manufacturers in design and attitude. Their PF series respirometers are based on a "pulse-flow" technology that involves the use of highly sensitive pressure transducers for detecting small increments of oxygen uptake or gas evolution from biological reactions.


Respirometer Systems and Applications (RSA) have considerable experience with the design and use of respirometers for analysis of the aerobic, anoxic, and anaerobic biological reactions that are critical for successful treatment of municipal and industrial wastewaters. Together, their team members and associates have over 90 years of experience with respirometers including three patents, over 100 publications, and a book on their design and application. Dr. James Young, a partner at RSA, is one of the patent holders on three of the respirometer systems that are produced today, and has over 45 years of experience in the field of respirometry. Bill Moore, the Marketing Director at RSA has sold and installed respirometer systems for almost 20 years. Another partner, Mike Ruff, has been designing respirometer systems for over 15 years. So RSA know respirometers and their applications better than others.


The RSA PF-8000 respirometer is the latest technology and is able to read in lower and higher ranges than any other respirometer. Right now RSA have clients who are reading over 100mLs per minute anaerobically with no calibration errors. No other system can do that. RSA's respirometer is also warranted for three years because of their belief in the system.


RSA's systems are simple to operate and have little to no maintenance. They always endeavour to be available for questions, so contact them for any queries you may have about their applications or specific set-ups for defined testing.


Reader Reply Card No. IET November / December 2012 www.envirotech-online.com 89


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