Controlling Microbiological Status of Activated Sludge
There is one area of paper industry microbiology where active measures are needed to cultivate microbial flora: biological waste water treatment plants.
When focusing on activated sludge of aeration basin (anaerobic treatments will be discussed in another post), the roles of different microbes should first be understood.
The tasks of bacteria and protozoa – which are the beneficial microbial groups in the purification process – are different: bacteries are responsible for the transformations (mineralisation) of incoming organic compounds, Protozoa collects small particles (also bacteria) and acts as indicators of the activated sludge condition.
Bacteria shall also build up “flocs” which can settle down and be either returned backwards into the beginning of the purification process or totally removed from the plant.
Significant amounts of nutrients will also be lead to the “sludge route” which is especially important for environmental reasons: no matter relatively low concentrations of nitrogen and phosphorus (compared with municipal waste water effluents), the volumes of P&P waste water effluents are huge.
Traditional parameters which are related to the microbiological status of activated sludge are C:P:R ratio, temperature, pH and oxygen concentration. In addition, the share of Protozoan indicator organisms are checked by microscopical methods.
ATP Assay is also a fast and reliable method to check the viability of activated sludge and control unfavourable sludge escape from secondary sedimentation basins (IM has written his second graduate work about this issue on 90’s).
Incubations with PMEU equipment (FINNOFLAG Oy, Finland) have been very successful when the effects of low oxygen concentrations on the structure of flocs are the subject of the study.
This method, in connection of bright field/dark field/phase contrast microscopy, gives also fast responses (even in hours) when the detrimental effects of toxic water fractions on activated sludge flora shall be checked.
There are many reasons for the bulking of activated sludge: nutrient supply, oxygen concentration, temperature etc. In all cases, a layer of activated sludge will rise on the surface of the basin and cannot be taken away from the water flow to sedimentation stages.
Measurements of redox potentials can give more information about the risks of anaerobic growth in biological waste water purification systems than sole oxygen concentration analyses (practical reslusts by IM).
RO potential control may also work as a replacement/addition to traditional Respiratory Rate test, often used to check the viability of activated sludge.
Microbial activities continue also in sedimentation basins. IM has detected a significant rise of nitrogen concentration in certain basins in studies performed on 80’s. The reason is obviously the activity of a coliform species, Klebsiella pneumoniae, which is able to fix nitrogen from atmosphere (nitrogen fixation).
The routes of the pathogenic Legionella pneumophila in the purification process should also be investigated more deeply. Novel types of analytical methods to replace the traditional colony count methods (intended in the control of household water) shall be developed, however: L.pneumophila is very hard to “find” among other bacteria and fungi which are capable to grow extensively on selective nutrient media for Legionella.
ON LINE control has – until now – not got any role in HACCP of waste water treatment systems but promising technical solutions are already available for semi-continuous control of activated sludge microbiology.
Juha V. Mentu – Pulp, paper, board and packaging microbiologist – July 2008
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