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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.
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