THE BASIS OF SEWAGE TREATMENT WORKS DESIGN
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THE BASIS OF SEWAGE TREATMENT WORKS DESIGN BIOLOGICAL FILTRATION PROCESS DESIGN 1. Introduction This lecture only covers the basic process design of biological filtration and concentrates on carbonaceous oxidation of domestic sewages. Information given in the paper has been gathered from various sources, therefore it should be fairly typical of that used throughout the UK. However, most companies have their own standard designs and standard details, resulting in possible minor variation with course members individual experiences to date. 2. The Process 2.1 Definition Biological filters are fixed film reactors which are colonised by aquatic aerobic micro organisms and moisture loving invertebrates. Settled sewage is distributed uniformly over the upper surface and trickles through the bed to the underdrains. The organisms in the film bring about oxidation and clarification for the sewage. The film is continuously growing and breaking off from the support media into the flow producing a sludge (humus) which is separated from the effluent by settlement in humus tanks. 2.2 The Biology of Biological Filtration Bacteria are the predominant group, numbers up to 2 x 109/ml. a) Heterotropic Bacteria This group characterised by a requirement for complex organic matter as a source of energy.They are responsible for the primary oxidation of pollutants. b) Autotrophic Bacteria These bacteria utilise carbon dioxide as their carbon source and inorganic chemicals as sources of energy for cell growth.They are principally represented by nitrosomonas (oxidising ammonia ion to nitrite) and nitrobacter (oxidising nitrite to nitrate). They grow as a zoogloeal slime on the fliter media. The volume of slime and numbers of bacteria diminish in relation to the food and energy supply downwards through the filter. The type of bacteria also varies throughout the bed with heterotrophic bacteria predominating in the upper part of the bed and autotrophic nitrifying bacteria present near the bottom. The food supply is mainly organic material in suspension or colloidal solution which becomes absorbed onto the surface of the slime. There it is rendered soluble, if not already so, and passes by diffusion into the bacterial cells. It is suggested that there is little difference in efficiency at the bacteria level over a sewage temperature range of 10C to 30C. This is probably due to changes in the species composition with temperature change. There are marked seasonal changes, however, in the efficiency of trickling filters due to disturbances of the film grazing fauna. Below 10C, temperature plays a direct role in changing their efficiency particularly with respect to nitrification. Fungi As they are heterotrophic microorganisms, when present they compete for the same food as the bacteria. They occur in most filter beds and can occasionally dominate the community often resulting in ponding. They can tolerate a high carbon : nitrogen ratio and more acid conditions than bacteria. They may outgrow bacteria in low winter temperatures, and at high substrate concentrations in the upper layers of filters. They metabolise a higher proportion of substrate than do bacteria. Numbers are about one eighth of bacteria in terms of volume. Protozoa These are a diverse group of unicellular organisms including swimming, stalked and crawling ciliates, flagellates and amoeba. The ciliates and amoebae consume the freely suspended bacteria in the liquid film and so clarify the effluent. Numerically protozoa are second to the bacteria, and although their numbers increase in winter there is no apparent effect on the effluent quality. When protozoa are sparse (e.g in conditions of overloading, after toxic shock, or high rate filtration) the filter effluent will be expected to be turbid). Metazoa These include successively rotifers, nematodes (non segmented round worms), annelidworms (segmented), dipteran flies (characterised by one pair of wings, many having aquatic larvae), beetles and springtails. Collectively they have often been referred to as “grazing organisms”. Nematodes feed mostly on particles and thus compete with protozoa. Annelid worms and insect larvae feed on the smaller film organisms in the lower layers of the filter and are most active in the summer months. They are responsible for controlling the extent of the film growth during the summer – the principal effect being the reduction of sludge production and increase in sludge settleability. During the winter their activity virtually ceases. In heavily loaded filters this can result in excessive film accumulation near the filter surface where the rate of slime growth is highest with consequent partial blockage leading to puddle formation (ponding). In modern filters the likelihood of this form of ponding is reduced due to the downward trent in filter organic loading. There are two periods during the year when the filters loose a large amount of film. This is known as sloughing and it is most noticeable in spring although there is usually a second lighter slough in later summer. Food Pyramid The feeding relationships between the various feeding (trophic) levels or organisms in a filter are shown in the diagram below. The arrows indicate transfer of matter between the various levels of organisms. The conversion of soluble and dead organic matter in the influent to mineral salts and degraded organic matter in the effluent is indicated. Humus sludge, consisting of surplus organisms washed out of the filter is also released. The pyramid represents the decreasing amount of organic matter transferred to higher levels as organic carbon containing compounds are utilised in respiration processes with the release of carbon dioxide and water. Flies Fly larvae & worms Rotifers and Nematodes Holozoic Protozoa Heterotropic Bacteria and Fungi Influent plus Saprobic Protozoa Dead Organic Solids Soluble Organic Wastes Degraded Organic Matter Mineral Salts Effluent 2.3 Chemical Requirements of Biological Filtration The chemical requirements are the same as for any aerobic biological process, i.e. oxygen nutrients in the ration 100 BOD : 5 nitrogen : 1 phosphorus neutral pH, 65 – 8.0 is preferred. It is the rate for any of these requirements to be limiting in a well designed filter treating domestic sewage. When trade wastes are present in relatively large proportions they may cause problems e.g wastes from brewing, distilleries, vegetable processing, paper making are deficient in N + P and chemical addition may be required. 2.4 Methods of Operation a) Single Filtration settled sewage Effluent Filter Humus Tank b) Recirculation settled sewage Effluent Filter Humus Tank Inlet Recirculation is used to control ponding due to excessive film growth, however is usually reduces nitrification. It may also be used to dilute strong wastes or to maintain the wetting rate at low flows in order to utilise the full volume of the filter bed. Care should be taken when employing recirculation that benefits gained by improving treatment on the filter are not lost by overloading the humus tank. A method of avoiding this is controlling recirculation according to the works inlet flow so that as the flow increases recirculation decreases. An alternative method is to recirculate filter effluent but this may not be satisfactory during periods of filter sloughing if the media is small. At small works it is sometimes beneficial to recirculate the flow to the primary tank inlet as this keeps the primary tanks fresh during periods of low flow. Recirculation is more expensive in capital and running costs than single filtration. c) Double filtration Settled Sewage Primary Second Final Effluent filter Intermediate filter Humus Humus Tank This method of operation gives a good performance although it is expensive in terms of capital and revenue costs. Its principle use is for strong wastes as it enables large media to be used in the first filter which will not block due to excessive film growth. The strength of the waste is reduced to a level where smaller media can be employed on the second filter. It is therefore more efficient per unit volume of media. The primary filter may be high rate plastic media or large (63 mm or more) mineral media. 2.5 Advantages and Disadvantages of Biological Filtration Advantages It is a low technology and therefore requires little technical supervision. It is a very robust process. Low revenue costs. Filters can become conditioned to some toxic wastes enabling them to be treated, e.g. phenols, cyanide. Disadvantages High capital cost Large land area required. 3. Process Design The main deign parameters are: Organic Loading Rate – Weight of BOD applied per day per unit volume of media (kg/m3/d). Hydraulic Loading Rate – volume of flow applied per day per unit volume of media (m3/m3/d). Both parameters are averages, usually annual averages. They therefore take into account diurnal and seasonal variations in load and flow. For a normal domestic sewage which does not contain excessive infiltration the organic loading rate will be the controlling parameter determining the effluent quality. However, as the hydraulic load on a filter increases the liquid retention period decreases, so that the flow is in contact with the film for less time and proportion of oxidisable matter removed decreases. Conversely, if the hydraulic loading it too low all the media may not be fully utilised or the film may dry out resulting in the filter not being used effectively. It is therefore important that the hydraulic loading remains at a controlled level. More consideration is now being given to the effective surface area of the media. The smaller the media the greater its surface area, thus more biomass is available to treat the sewage. Therefore the smaller the media, the smaller the spaces between the media resulting in less air circulation and a greater potential for blockage due to film accumulation. it can be seen that a balance has to be taken between the two opposing factors. Generally 50 mm media is used in filters and this provides the optimum surface area for most domestic sewages. However the volume of the filter can be reduced by using 40 mm media if the sewage is weak and a larger media must be used for strong wastes, unless it can be diluted by recirculation, etc. The importance of good data cannot be over stressed. Good analytical and flow data is essential in order to identify the load to be treated. Laboratory treatability/toxicity analysis will indicate whether the sewage is more or less treatable than domestic sewage and indicate the presence of toxic substances. The performance of existing treatment units or pilot plant data will enable more precise predictions of plant performance and lead to capital productivity as it will be possible to reduce the “comfort factor”, i.e the additional capacity included just in case. The loadings given below are for a typical domestic sewage on 50 mm slag media. 95% ile Effluent Standard Organic Load Maximum SS BOD NH3 kgBOD/m3/d Hydraulic Load at DWF m3/m3/d 60 40 0.12-0.15 1.2 45 30 10 0.09-0.12 0.6 *30 20 5 0.07-0.09 0.6 The range is due to sources of information being used. * It may not be possible to achieve this standard without tertiary treatment. Over the years the recommended organic load to achieve a particular effluent quality has reduced. This could be due to increasing knowledge of the performance of filters but my be more due to the greater requirement to comply with a legal consent.