Techniques to control industrial wastewater

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.