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Waste water treatment is not quite the same as treatment of the sick. For humans a visit to the doctor is usually prompted by one, perhaps two presenting complaints. After diagnosis, the Doctor will look at the patient and say in a grave voice – you have small pox, but you will be fine. Just take this course of anti-biotics and get plenty of rest. For waste water treatment the list of presenting complaints is often a big list. For instance the presenting complaint might be prompted by an irate neighbour complaining of smells or a Municipality charging huge surcharges.
On analysis the waste water can be suffering from low pH, high conductivity, high COD and high turbidity. In almost all cases there will not be a single cure for each of these conditions. Each of these parameters will require it’s own set of treatment processes. If biological treatment is deemed the appropriate route, some of the out of specification parameters will have to be addressed first. So here we have a patient who not only has small pox, but also has black eye, broken ribs a nasty cold and a bout of shingles.
One of the big challenges in the treatment of waste water is COD reduction. COD stands for chemical oxygen demand and is a measure of the dissolved organic material in a waste water. It is in some ways analogous to calories and kilojoules – terms used for describing the potential carbohydrate benefit (or not) of food stuffs. COD can be biodegradable or not, it can be slowly biodegradable or rapidly biodegradable. The COD test measures the entire range – biodegradable, unbiodegradable and everything in between.
The reason that COD is difficult to treat is because it is soluble in water. It does not filter out very easily – abit like sugar in coffee. At the reverse osmosis level COD reduction is possible but this a clumsy method of COD reduction because the membranes will foul quickly. Some forms of COD can be teased out of solution via flocculation and sedimentation. There is always the option to boil the waste water and remove the evapourated and pure water in a distillation column. This is an extremely energy expensive method and is generally not considered viable.
A large amount of waste water types have COD concentrations that are very definitely biodegradable in character. Pretty much any waste water that is generated from a food or beverage producing facility will contain significant amounts of biodegradable COD. And why would we want to remove biodegradable COD? In a nutshell, water borne bacteria like biodegradable COD. Bacteria is around us all the time and they have an uncanny ability to locate and colonise any available food source – even food sources that are not very viable. The bacteria do a fantastic job in degrading COD, converting it to biomass, gas, heat and water. Untreated waste water with concentrations of biodegradable COD will attract bacteria in large numbers, simultaneously creating and solving problems. The big by-product problems are smell, clogging of soils and depletion of dissolved oxygen in rivers. These are the issues that will annoy a neighbour or notch up Municipal surcharges.
Removal of COD using bacteria requires mimicry of natural systems. The biological process requires time, balanced pH, sufficient nutrients, enough dissolved oxygen and minimal presence of toxins. This is a process that cannot be rushed and if designed with care will provide unsurpassed service. As clever as we like to think we are, engineers have not been able to mechanize biological processes. Until this happens we have to be nice to bacteria, really nice. (Thanks to Bob Hadley for coining this phrase)