Techniques to control industrial wastewater
The number of industries which choose to treat their waste on-site prior to discharge to either sewer or surface water continues to grow. Dr Nigel Horan of Aqua Enviro at the School of Civil Engineering at the University of Leeds explains some simple techniques for the control of industrial wastewater treatment plants.The number of industries which choose to treat their wastewater on-site prior to discharge to either sewer or surface waters, continues to grow. This is being driven both by the increasingly stringent discharge standards imposed by the Environment Agency and by water companies who, in an attempt to improve the quality of their own effluent, go Œup-the-pipe¹ to identify discharges which are likely to prove difficult to treat.
However there is one big similarity between treating an industrial wastewater and a domestic one, namely that both types of treatment rely on microorganisms and the requirements of these organisms are the same. The organisms that play the biggest role in wastewater treatment are the bacteria, with the protozoa playing a smaller but nevertheless essential role. These organisms have requirements and behaviour patterns very similar to our own. They like a source of digestible food served at the correct temperature and with the appropriate nutrients. Once they are well fed they like nothing better than reproducing and dividing to increase their numbers. Providing they are given the right food source and an adequate amount of time to reproduce, they will reward the plant operator by providing trouble free and effective wastewater treatment.
The ability of microorganisms to degrade a particular waste stream is expressed as an organic removal rate, for instance, 0.2kg BOD removed per kg microorganisms per day. This value is determined typically from a treatability study and forms the basis of plant operation. Obviously if a plant receives a feed greater than the rate it can remove it, then the excess will appear in the effluent. Thus the food to microorganism (F/M) ratio is a key operating parameter. The amount of time an organism must reside in the reactor to digest the food and reproduce is also determined from a treatability study, and it is known as the sludge age. So providing that the F/M ratio and the sludge age are at an optimum for the waste under treatment and providing that the wastewater contains adequate nutrients such as nitrogen, phosphorus, trace metals and vitamins, and with adequate aeration to ensure that the dissolved oxygen does not fall below around 2mg/l at any point in the aeration basin, the plant should be performing perfectly.
But how does an operator know that the sludge age and the F/M are at an optimum for a wastewater that has a continually changing composition? By far the easiest way to find this out is to look at the organisms themselves and find out if their is a correct balance of microbial species in the wastewater, in other words conduct a microscopic examination. Because of the small size of bacteria it is not possible to identify the majority of them by microscopy. However the protozoa can be seen and identified with ease using a relatively inexpensive microscope and the type of protozoa present is a clear indication of the operating conditions of the plant. A very simple scheme that can be used by any plant operator divides the protozoa up into four easily identifiable groups, namely: flagellates, free-swimming or stalked and rotifers (although the rotifers are not actually protozoa but metazoa) (figure 2).
The flagellates are small and very fast moving. They require a lot of energy which they get from the soluble organic matter contributing to the BOD. Thus if a lot of free-swimming protozoa are present there is a high BOD, indicating a plant that is receiving an excessive loading. Free swimming ciliates are much larger and slower swimmers and are very efficient feeders. As the organic matter/BOD declines, they will out-compete and replace the flagellates. Their presence indicates an improved effluent quality, but with a loading rate still on the high side. As the loading rate is reduced further, there is little food available but a large number of bacteria, which used the initial conditions for growth and division. Thus the stalked ciliates are able to colonise, feeding on these bacteria.
This is an ideal situation with the bacteria reducing the BOD and the protozoa removing the bacteria, to generate a clarified effluent with a low BOD and a well-settling sludge. A plant operator who sees something resembling figure 3, when looking down a microscope, can be confident they have a well operated and optimised plant. Finally as the loading is reduced even further the rotifers start to appear. These metazoa are large, slow moving and able to consume both bacteria and protozoa. They are typical of plants with long sludge ages and low loading rates. They are found in large numbers in extended aeration systems.
It is relatively easy to record the numbers of each of the four groups of protozoa each time a sample of mixed liquor is examined. If this data is stored along with information on the F/M ratio, the sludge age and the effluent BOD, it is possible to plot the numbers of protozoa against these operating parameters. This is termed a Relative Predominance Diagram (RPD). Once enough data has been gathered, the RPD becomes a valuable tool for predicting operating conditions based on microscopic examination. Perhaps more importantly it can be used to actually control the plant to adjust the operating conditions to obtain the protozoal populations that have been shown to provide the best effluent quality.
Routine microscopic examination is a valuable tool for efficient plant
operation, and the majority of treatment plants have a suitable microscope
purchased for just this purpose. Unfortunately proper use of this facility
can be a daunting task for most plant operators who do not have a biological
background. In an attempt to make this option more widely used, a simple
laminated wall chart has been prepared which details the procedures to be
followed in undertaking a microscopic sludge examination. The Activated
sludge troubleshooting chart has been prepared by Aqua Enviro at the
University of Leeds in conjunction with HydroCare HydroChemicals (UK). It
provides examples of the important protozoa and filaments which are
routinely encountered in activated sludges and it also explains clearly how
the results of microscopic examination should be recorded and interpreted.
Copies of the new Activated sludge troubleshooting wall-chart can be
purchased for £15 from Aqua Enviro at Aqua Enviro/HydroCare, c/o Hydro
Chemicals (UK), Immingham NE Lincolnshire, DN40 2NS.