Case Studies

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Waste water treatment activated sludge Common problems.
There are many types of waste water treatment. All are good at what they do but not every type is suitable for every waste. When deciding what treatment is best for a type of waste many things have to be considered.
Dairy and food production effluent, although is not difficult to treat has high organic load together with high volumes. this eliminates many types of treatment as the flow through the plant would be too fast for the system, or the organic loading would not have long enough contact time to be removed. This is why activated sludge, is often used for this type of waste stream.
Activated sludge plants are a very effective method of waste water treatment. They are capable of removing up to 99% of organic matter. This is if conditions in the plant are kept to design parameters, with a consistent influent.
Changes in influent (incoming waste stream) can seriously affect the efficiency of treatment, leading to various problems. These can include poor BOD removal, bulking sludge, high ammonia, and solids carryover, Filamentous bacteria formation, foaming, trouble meeting your Consent limits leading to problems with the Environment Agency or other bodies who grant the discharge consent for the site.
Changes to the influent of a plant can cause Overloading, Toxic shock, starvation, all of which provide poor quality final effluent and very sick plants.
The other main causes of a poorly performing plant are Poor education of operator, Poor plant operation control, No adequate testing and/ or Plant design.
To consider common plant problems the following two case studies show two typical examples of poorly performing plants.
Case Study 1 of a vegetable preparation factory in Brecon Powys.
The treatment of the Waste water from the factory recently ran into problems. An activated sludge plant is use to treat the trade waste. For no apparent reason instead of the usual excellent final effluent quality, a grey smelly final effluent was being produced. The final effluent contained copious quantities of free sludge which did not settle and was being discharged with the final effluent. The treatment plant manager tried all the usual tricks to improve the situation. Nothing seemed to be working. The discharge to water course had to be stopped, with various avenues explored with no success.
A prompt onsite visit was needed, to assess the situation. During the visit to the plant several onsite tests were run. The final effluent was indeed grey and smelly with huge volumes of dead sludge suspended in the final effluent. On inspection of the sludge it was evident that not all the sludge was dead, with a good viable solid content interspersed with dead and dying solids. This all pointed to a condition of toxic shock. There was or had been something in the influent that had had a toxic effect on the bio-sludge. After talks with the plant manager the operator and the factory concerned, the source of the problem was difficult to exactly identify. The treatment plant is shared with another user. Neither party was willing the admit that there was a problem from their side. The problem of getting the treatment back and resumption of discharge was vital. After following the suggestions that were made, the manager of the plant saw an immediate improvement in treatment and was able to resume discharge within twenty four hours. Further investigations discovered the suspected source. To prevent a reoccurrence of the problems, new working practises were suggested for the factory, and stricter supervision of the waste stream and possible contaminants recommended for the other party
Case study 2 of a cheese producing factory in Cheshire
The plant had had a chequered history of efficient running. Swinging from excellent treatment with final effluent well within consent limits to practically no treatment of an extremely high BOD loaded influent.
In this case there was a lack of knowledge, about how an activated sludge system works. In many attempts to gain the requisite knowledge, many 'experts' were called in. the biggest problem was that they were all in business to sell products. These 'experts' all offered solutions but used in combination they were disastrous
. Once an independent consultant was brought on to assess the problems and causes, it was clear that all the advice of all the 'experts' had been listened to but, as so much of it was conflicting there was total confusion
. The other main areas of potential problems were found to be inconsistent waste from the factory. This ranged from extreamly high loadings to very week. The pH was found to be very acidic with a hap-hazard method of pH correction. There was no oxygen monitoring. There was no on site testing available, so the varying strengths of the effluent often went undetected. This meant that the operator was running the plant 'blind'.
After consultation with the management, plant operator, engineer and consultant a plan was formulated of education, plant design modification, waste stream survey and improved communication between departments. This all took time but eventually, the plant was able discharge a consistent good quality of effluent. The operator received sufficent training to gain a sound understanding of how and why the plant worked. Potential problems were highlighted and understood. This was backed up with support from the consultant. The backup and support is an on going process, with all parties working together to achieve consistent effluent with out compromising the business need of the factory.
The science that makes it work In the two case studies both started with the same problem. An activated sludge plant that was not performing. But both had separate and different problems to address. In the following discussion I hope to go some way to explaining, the conclusions that were drawn and the solutions offered. In order to do this the reader should understand a little background of how an activated plant works. The following gives a brief summary, of the science behind the application.
Activated sludge plants put in their simplest terms are tanks of bacteria that use waste water for their food and oxygen to live and grow.
The sludge itself is made up of common bacteria that are often found in soil or nature. Each type of bacteria has a specific purpose e.g. nitrifying bacteria, this bacteria is not one single strain of bacteria but a family with several members that each has its part to play in breaking down organic nitrogen from solid to liquid to gas. This family of bacteria work best under specific conditions. The activated sludge is made up of many families all with their part to play. Also found in the sludge are single celled organisms called protozoa again not one family of protozoa but many.
For every waste stream there is a designer sludge, this grows and is constantly adapting to the needs of the waste, as every waste is different. Once the designer sludge has been established it happily treats that waste stream. Problems arise when the waste stream changes, usually because an extra element is introduced that no bacteria are present to use. Oxygen is essential for the well being of the bacteria, if the waste is to strong the bugs (activated sludge) use vast amounts of dissolved oxygen, conversely when the effluent is weak little dissolved oxygen is required. Therefore managing sludge can be very difficult if the waste stream is constantly changing.
An activated sludge plant has several stages of treatment; the following flow diagram gives a basic outline to this. This is a basic design. Various extra treatments can be put in line to enhance treatment; such as a dissolved air flotation (DAF)plant, intermediate settlement, an anoxic tank, sand filter etc
Once this is understood, the other constituent parts of the plant have to be understood, and the knowledge applied to get the plant to run.
Often running an activated sludge plant is referred to as a black art. There may be some truth to this but generally it is not magic, it is simple common sense backed up with experience and understanding.
For a plant to work well, there are many factors to take into consideration. The size of the treatment plant has to be of adequate size. In sizing a plant several factors have to be considered. These are the volume of waste water, the strength of the influent, (organic loading) the amount of available dissolved oxygen and most importantly the consent level.
The dissolved oxygen must be sufficient to provide the correct amount of oxygen required by the bacteria. Too much oxygen can be as big a problem as too little.
Consent levels. The level of consent is set by the receiving body. If this is a water course the consent is set at the amount of volume that the water course can accept, with out affecting the physical levels within the water course. The biological oxygen demand, the ammonia, the phosphate, or other nutrient, heavy metals etc. are calculated so the impact on the receiving water will not, have an adverse effect on the nutrient balance, within the water course. If the treated effluent is to go to sewer for further treatment the consent limit is determined by the volume and nutrient content that is acceptable to the treatment works. Consents are very important with prosecution being the penalty for breaches. These usually result in heavy fines for the offender and can lead to imprisonment. Consents are set to minimise pollution, with a well treated effluent the risk is minimal, however, poor treatment is potentially a big pollution risk. Pollution in Brittan's waterways has been drastically reduced in recent years. This has had a big environmental impact with native species of fish and aquatic mammals returning to areas where they have not been seen, for a very long time. It was thought that some of these species had died out.
In case study 1 the problem was not lack of knowledge, plant design or available oxygen but a significant change in the incoming influent. The cause was found to be a significant increase in a disinfectant/bio-side that had been introduced in the cleaning regime of the factor. The effect on the effluent was to kill off the sludge. The change in the factory had been made without consulting the treatment works manager. Had the manager known that the chemical was to be used the effluent could have been treated differently. The effluent from that particular part of the factory could have been isolated and gradually introduced to the sludge. By doing this the sludge would have had the time to adapt and grow sufficient amounts of the family of bacteria that can treat the change in effluent. In the short term the contaminated supernatant had to be removed and taken away.
Once the serious nature of the over use of the disinfectant was explained to the management of the factory, a course of action and safeguards were put in place. This not only made the running of the plant easier but the company was able to save considerable amounts of money on the reduced amount of expensive disinfectant.
In case study 2 The problems were the same but the cause was different. It took time to find the cause.
It emerged that changes to the operation of the plant were being made on a frequent basis, with a mixture of additions. The pH dosing was constantly being changed; polymer addition was in use without proper control, the strength of the effluent changed on a daily basis, there was no oxygen control or indeed any monitoring at all. No routine testing was being carried out to monitor the variance of performance; the plant operator had no training and was unable to get the basic information that was needed to run the plant. The answer to the problem was more difficult and took much longer as a whole re-education programme had to be introduced. Stability had to be established in the treatment. Routine testing was introduced so that a better picture could be drawn. The operator had to be trained to recognise changes in the effluent hand how to adjust the treatment accordingly.
Regulation and control of the oxygen was introduced. Communication was improved between the management and the operators to notify when problems in the factory happened; this meant that appropriate action could be taken at the effluent plant.
In conclusion.
For an activated sludge treatment plant to work efficiently, consistent influent is vital. To operate the plant it is important that the operator has at least a fundamental understanding of how a plant works and the common causes of problems. Too often quick fixes are sought rather than looking further to find out what is causing the problem. Often activated sludge plants are used to treat domestic sewage and industrial trade waste. In industry the priority is to get the product out of the factory gate, with little or no regard of the impact on the treatment plant. Food factories must be clean therefore there has to be a high use of chemical cleaners often a change in one of these products can have a devastating impact on the effluent treatment. Sloppy work regimes where operators are under pressure, there is often a disregard of what goes down the drain. One very important consideration in treating trade waste is that the production must be profitable. Communication and team work are key to making the factory profitable and the effluent within the consent limits set by regulatory authorities.
By Julie Hallam April 2004
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