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                         INDUSTRIAL WASTEWATER 2
                           BIOLOGICAL TREATMENT
        Meng/MSc Dr R Stott Tel: 2393 Email Stottr@civl.port.ac.uk
                            14th October 1997

(A collection of handouts is available if required)

INTRODUCTION

Within the UK, there are a wide variety of industries producing wastewaters that
require treatment. Current estimates suggest that in the UK, industry produces 7 x
106m wastewater per day.

Depending on the nature of the industry, wastewaters can contain a wide range of
pollutants but in general, industrial wastewaters contain the following:

soluble organics            deplete DO
suspended solids            deplete DO and release undesirable gases
Trace organics              affect taste, odours and toxicity
Heavy metals                are toxic
Colour and turbidity        affect aesthetics
Nutrients N and P           cause eutrophication
Refractory substance        resistant to biodegradation/toxic to aquatic life
oil and floating substances unsightly
volatile substances         H2S and other VOC cause air pollution

Pollutants of importance include non metals such as arsenic, heavy metals,
Heavy metals are found in wastewaters from mining and ore processing industries,
copper found in Electro plating industries,

The levels of the constituents above in effluents are governed by EU directives.
Typical pollutants and BOD range for a variety of industrial wastes are given below.

                   Wastewater characteristics for typical industries

INDUSTRY                    Principal pollutants                   BOD5
                                                                   mg/l
Dairy, milk processing      Carbohydrates, fats, proteins          1000 – 2500
Meat processing             SS, protein                            200 – 250
Poultry processing          SS, protein                            100 – 2400
Bacon processing            SS, protein                            900 – 1800
Sugar refining              SS, Carbohydrates                      200 – 1700
Breweries                   Carbohydrates, protein                 500 – 1300
Canning fruit etc           SS, Carbohydrates                      500 – 1200
Tanning                     SS, protein, sulphide                  250 – 1700
Electroplating              heavy metals                           minimal
Laundry                     SS, Carbohydrates, soaps, oils         800 – 1200
Chemical plant              SS, acidity, alkalinity                250 - 1500
SS -= suspended solids


The levels and nature of the contaminants can vary depending on whether batch or
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continuous production of products is carried out. These pollutants must be removed
or reduced to acceptable levels to satisfy discharge consents in order to protect
aquatic ecosystems in receiving waters. The impact of a compound on the aquatic
environment and therefore, whether it can be discharged safely to that environment
depends on
       amount to be discharged
       extent of degradation
       toxicity to flora and fauna etc.
       bioaccumulation

Because of the variety of contaminants, a wide range of treatment processes have
been developed to treat industrial wastewaters and hence limit their impact on
aquatic ecosystems when discharged into receiving waters.

There are many treatment methods available for the treatment of Industrial wastes.
Physical and chemical processes have already been discussed in a previous lecture.
However, biological treatment is still the most commonly utilised method for the
treatment of Industrial wastes.

There are several factors that affect the selection of a particularly biological process
including site specific constraints, flow and load balancing but of major importance
are those factors affecting the performance of biological treatment itself such as
biodegradability of wastes and favourable biochemical characteristics such as C:N:P
ratio (i.e. nutrient imbalance), pH etc and toxicity of pollutants in the wastes.

BIODEGRADABILITY

If organic compounds are not biodegradable, it is unlikely that they will be removed
by conventional sewage treatment systems and will be present in the receiving
waters and consequently result in pollution effects Implications for pollution include
toxic impact on aquatic life as well as imparting undesirable characteristics to the
receiving water such as colour, turbidity, taste, odour or tendency to foam. In
addition, accumulation of non biodegradable toxic compounds in sludges present
problems for land disposal since toxicity may affect soil fauna and flora..

Organic compounds that are slowly degraded by bacteria are called
RECALCITRANT or REFRACTORY. E.g. pesticides, insecticides and herbicides.
These compounds can survive for extended periods in the environment.

A great variety of compounds present in industrial wastewater may be biodegradable
or non biodegradable. Biodegradable compounds include
alcholols, acids, fats, oils and greases, NH3, NO3. Non-biodegradable compounds
include pesticides, PCB, metal chlorides and metal sulphates. Surface active
agents used for washing etc., may be degraded very slowly.

The biodegradability of a compound is influenced by its molecular size, solubility,
and chemical structure. These all increase the resistance to degradation. long
chain carbons molecules are easier to breakdown than complex double bonded or
helical structures. Synthetic organic compounds can contain strong chemical bonds
which may resist microbial degradation.
Other Factors affecting biodegradation include

   nature and concentration of microbial population
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   concentration of test substance (may be inhibitory)
   nutrient availability
   temperature, pH DO concentration

Therefore, before discharge of wastewaters to sewers or process selection, it is
important to establish the biodegradability of the wastewaters. This information is
important for selection and design of treatment system and need for additional
treatment units in order to facilitate its degradation. eg. Increase oxygen may be
required, larger volumes of sludge may be produced, higher concentration of
suspended solids may have to be carried in the mixed liquor and physical properties
of the activated sludge may change.


TESTING SUITABILITY OF WASTEWATERS FOR BIOLOGICAL TREATMENT

There are a variety of methods available for assessing the biodegradability of
wastewaters. But to analyse for a whole suite of compounds may be very difficult
and to characterise the wastewater might be very tedious and costly. Therefore we
can test indirectly by assessing oxygen uptake or directly by simulating various
conditions (e.g. Hussman and Porous pot).

1      Biodegradability tests

A     COD/BOD ratio
One of the most common methods is to assess the COD/BOD ratio of the
wastewater. (A laboratory practical class will be using this method).

BOD is measured as biodegradable organic carbon. The method measures the
amount of dissolved oxygen used by microbial activity in the biochemical oxidation of
organic matter (i.e. the biochemical oxygen demand)/ Toxic compounds inhibit the
activity of micro-organisms which degrade organic matter. IF these compounds are
present in wastewaters and we test the BOD, the activity of the organisms in the
BOD test bottle is reduced , therefore there is less uptake of dissolved oxygen since
oxygen requirements are reduced. Thus the BOD is suppressed.

The COD (Chemical Oxygen Demand) measures the total organic carbon (bio and
non-biodegradable) in wastewater by chemically oxidising the compounds. It also
provides an indication of the presence of toxic compounds.

The ratio of COD to BOD can give an indication of the biodegradability of a
wastewater.
I      In domestic sewage which is known to be readily biodegradable and treated
successfully world-wide using a variety of biological treatment methods, the
COD/BOD ratio varies typically from 1.5 : 2.

Thus if the COD/BOD ratio of industrial wastewater is also < 2 , this provides a
good indication that the wastewater can be treated biologically.

ii      If the COD/BOD ratio is high and generally >5:1 , this indicates that the
wastewater is non biodegradable , to xicity or nutrient imbalance is present and thus
will present problems if biological treatment selected.
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iii   If COD/BOD falls between 2 – 5 :1 then this is a grey area. A lot of industrial
wastewaters fall into this category . Therefore, further studies are needed.

Typical COD/BOD relationships are outlined below for a variety of industrial
wastewaters.

          Wastewater              Influent             Effluent         COD/BOD
                               BOD       COD         BOD      COD
                               mg/l      mg/l        mg/l     mg/l
        Pharmaceutical         3290      5780         23      561          1.8:1
           Tannery             1160      4360         54      561          3.8:1
           Tobacco             2420      4270        139      546          1.8:1
       Vegetable tannery       2396     11663         92      1578         4.9:1
          Textile dye           393       951         20      261          2.4:1

In some cases, it is sufficient to determine COD/BOD ratio. However, the test
provides little information on the potential of micro-organisms to acclimatise. If
analysis suggests that BOD is very low, then it is advisable to assess the toxicity of
wastewater to bacteria so that a non-inhibitory concentration of the sample can be
used for further biodegradability test and that the potential of Micro-organisms to
acclimatise and eventually breakdown the compounds can be assessed. .


b      Simulation tests for biological processes

Simulation tests can be performed that mimic treatment processes. These test
whether there are micro-organisms capable of degrading the wastewater under
conditions of that test. An innoculum is used so that a wide population of numerous
species are introduced so that one or several species capable of degrading the
wastewater or can adapt to do so.

General conditions for each test are
 adequate concentrations of nutrients available
 Temperature (18 - 25C)
 pH ( 6.5 - 8.5)and
 DO values (> 2 mg/l) should be conducive to good growth

These tests thus have limitations in that they are carried out under defined and
optimal conditions that may not favour biodegradation in a full scale plant.



i:      Respiration rate test
Respiration methods measure the degree of oxygen uptake by micro-organism .
It is advisable to do this test first to assess the toxicity of wastewater so that later
tests can be carried out at non-inhibitory concentrations. There are several
respirometry methods available but a simple procedure measures biodegradability
by means of Oxygen uptake measured by a manometric respirometer.

Principles:
A known amount of test wastewater is added to a mineral medium and inoculated
with activated sludge and stirred in a closed flask. Consumption of oxygen is
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determined by the change in volume or pressure in the apparatus.
Blanks are prepared in order to determine “control” levels of respiration. During the
test, evolved CO2 (produced by the microbial degradation of organic material) is
absorbed in potassium hydroxide solution. Increased oxygen uptake obtained (over
a period of not more than 28 days) is compared with the COD.

Oxygen uptake values are determined as mg and results plotted against time. The
advantage of the method are that since the test is run over a long period of time (up
to one month) it can detect lags which can indicate whether micro-organisms could
climatise i.e. self select so that once climatised, biodegradation could occur.

Respirometry methods can also be used to test nutrient dosing and pH amendment
if these are required.

ii:     Continuous simulation test (Activated sludge)
There are several continuous simulation tests available that can be used to assess
biodegradability of industrial wastewaters and hence the suitability of biological
treatment. The international standard is the Hussman apparatus but Porous pot
methods may be an appropriate alternative method. They have advantages over
other methods in that they can test large volumes (eg litres) of wastewater. ( Mixed
cultures such as activated sludge and filter s treating mixed wastes are less susceptible to
toxic compounds than individual populations)

I      Hussman apparatus
The basis of the method is to determine the removal of a substance in a laboratory
scale plant resembling an activated sludge process. A maximum of 12 weeks is
required.

A few litres of wastewater can be tested (in this case) 3 litres is added to activated
sludge in an aeration vessel (Chamber C) at a rate of 1 litre/hour by dosing device
(B). Mixed liquor is settled in an adjoining vessel (Chamber D).
D has its outlet fitted at top at a level to maintain 3 litres in the aeration chamber C.
The Hussman unit has an air-lift pump (Device E) which returns settled sludge at an
abnormally high rate (>12:1 of the sewage flow) so anaerobic conditions do not
develop during settlement. Normally the return flow of sludge is nearly equal to the
inflow of sewage. Two diffusers (G) are placed in the aeration chamber to produce
fine bubbles and to achieve nearly complete mixing.


Influent and effluent are analysed for BOD and COD. Samples are taken twice a
week during the running in period (i.e. time taken to reach 2.5g MLSS/l and steady
removal of COD usually 78-88/%). It should not exceed 6 weeks.
Once steady level reached, samples are collected daily over a period of 3 weeks.
The biodegradability is expressed as the percentage removal

II     Porous pot

In contrast to the Hussman apparatus, the porous pot has no settlement period and
so no air lift. Most of the solids in the mixed liquor are retained in the aeration
vessel by porous walls. These units tend to be easier to run than Hussman units
and are easier for temperature control.

The porous pots are constructed from sheets of porous polythene 2 mm thick,
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maximum pore size 95 um). The porous pot is contained in an impervious PVC
vessel (Vessel D) and set into a controlled waterbath with a metered air supply to
base of pot (G) and a diffuser (F). The pump delivers 1l/hour to the pot with an air
flow about 2.5 l/min , temp 18-25C. MLSS is determined twice weekly and
maintained at 2.5 g SS/l.

Whilst both these methods have advantages over others, they are laboratory scale
and thus may not reflect the true conditions in operation in a large scale works.
Also, material that fails the biodegradability test by still be readily degraded.

2      Toxicity

Biological treatment is also susceptible to toxic effects since the toxic effect may
inhibit or reduce microbial degradation of organic material. Some organic
substances such as phenol are biodegradable at low concentrations but are toxic at
higher levels. Heavy metals accumulate in biological systems and so their toxicity
is affected by the treatment process used and plant operation.
Also, the accumulation of heavy metals can cause problems with sludge disposal.
Salts and ammonia, which are common to most effluents can inhibit biological
activity when at high concentrations.

Pollutants of particular concern in industrial wastewaters include
 heavy metals eg mercury, cadmium
 Organo-chlorine biocides
chlorinated industrial cleaning agents

High COD/BOD ratios can indicate a potential toxicity problem. If heavy metal
contamination is suspected, analytical techniques such as atomic absorption
spectrophotometry can be undertaken.
Metals such as mercury and copper form complexes with enzymes a nd other
metabolic agents concerned with respiration and therefore reduce or inhibit enzyme
activity thereby interfering with microbial degradation.

However, in addition to the inhibition of beneficial treatment processes , toxic
compounds in industrial wastewaters may also be toxic to aquatic life and so their
release into receiving waters has great consequences for aquatic ecosystems.

Heavy metals and Organo chlorine compounds can accumulate in filter feeding
organisms and so pass through the food chain to predator species in the trophic
level above. Very high levels of toxin can arise because of “biomagnification” where
the concentration of the compound accumulates in the flesh.

The sensitivity of organisms to toxic compounds may depend on a variety of factors
including temperature, pH, presence of other pollutants, and exposure time to the
toxin. The EC Dangerous Substances in Water Directive lists specific chemicals
that are under strict control and the Environmental Protection Act 1990 has produced
a red list of chemicals that are banned in effluents because they are extremely
toxic, persistent in the environment and have a great potential for bioaccumulation
through the food chain. (see lecture on Impact and detection of pollutants for further
information)

Therefore, the release of toxic compounds into receiving waters has great
consequences for aquatic life so toxicity tests may be conducted to assess potential
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toxicity of industrial effluents/waste streams that will be discharged.

Toxicity is commonly assessed using bioassays i.e. an aquatic organism typically
fish or daphnia are exposed to varying concentrations of suspected toxic pollutants
and the lethal dose or concentration at which 50% of organisms (i.e. LD 50, LC50
respectively) are affected over a given time period (usually 48 or 98 hours) is
established (refer to Impact and detection of pollutants for details on LD50 values).

Bioassays are also undertaken to establish the pollution tolerance of invertebrate
populations. Tests have been developed such as “microtox” in which an LD50 can
be established for groups of micro-organisms.

To overcome the toxic effects of compounds in the wastewater during treatment,
completely mixed treatment processes can be used. In these, the inlet
concentration of pollutants is diluted so that micro-organisms are only subjected to
effluent levels of contaminants.
Also, continued exposure to low levels of contaminants can result in acclimatisation
and the production of tolerant communities.


3      Nutrient imbalance

The micro-organisms responsible for biological treatment have certain nutrient
requirements which may not be fulfilled by industrial effluents. To break down
organic matter, the carbon, nitrogen and phosphorus nutrients are utilised in the ratio
of 100:5:1 (C:N:P). Rate limiting nutrients for growth of micro-organisms and
nutrient imbalance can result in a reduction of biological treatment so nutrient
supplements are required. Often industrial effluents require the addition of nutrients
such as phosphorus or nitrogen for effective biological treatment.

Trace elements of inorganics may also be required to supplement microbial growth.

                  Trace nutrient requirements for biological oxidation

                                         mg/mg BOD
                                  Mn     10 x 10-5
                                  Cu     14.6 x 10 –5
                                  Zn     16 x 10-5
                                  Mg     30 x 10-4
                                  Ca     62 x 10-4
                                  Na     5 x 10-5
                                  K      45 x 10-4
                                  Fe     12 x 10-3


Any of these nutrients can be rate limiting for the growth of micro-organisms if not
present in sufficient amounts. The imbalance in nutrients can therefore result in the
reduction in biological treatment due to a lower rate of breakdown of organics.
Limiting nutrients may also promote the growth of undesirable colonies such as
filamentous bacteria which can cause bulking of sludge and poor sedimentation.

Therefore, certain types of wastewater may require nutrient supplements in order to
main treatment performance.
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Thus to assess for suitability of biological treatment for industrial wastewaters a
range of tests might be advisable to investigate a whole range of compounds known
to exist in the wastewater.

TYPES OF BIOLOGICAL TREATMENT

Biological processes can be classified generally as aerobic or anaerobic. Biological
treatment is the most common method of treatment for municipal and industrial
wastewaters. Aerobic or anaerobic are widely used for the treatment of domestic
sewage and sludge but can also be applied to the treatment of industrial
wastewaters. There is a wide range of processes available but adoption will
typically consider cost and treatment quality objectives of the effluent. Two distinct
wastewater treatment policies have evolved depending on the country. Industrial
wastewater is treated independently of municipal wastewater in Ireland. In the UK,
industrial wastewater is usually treated in combination with municipal wastewater.

AEROBIC BIOLOGICAL TREATMENT SYSTEMS

AEROBIC process can be categorised as
 suspended growth
 or attached growth

a      Suspended growth
In suspended growth systems, a high microbial concentration is achieved through
recycling of biological solids. The principles of suspended growth systems are that
 bacteria convert biodegradable organics and some inorganics into new biomass
    and water and carbon dioxide.
 Biomass is removed as sludge
 liquid is settled and removed as clarified effluent
 Gases are air stripped

Types of suspended growth systems include
Activated sludge
Stabilisation ponds
Aerated lagoons
Containment ponds
Constructed wetlands

1      Activated sludge systems
Most treatment plants use activated sludge for the treatment of industrial
wastewaters. Operating procedures can be modified and so several types of
activated sludge systems are available.

Activated sludge systems comprises an aeration tank and secondary clarification
tank. The aeration tank retains influent wastewater for hours (maybe days) in a well
mixed aerobic environment. Settlement is carried out in the clarification tank and
about 20% of sludge is returned from clarification tank to aeration tank (RAS:
returned activated sludge). The sludge contains a high density of micro-organisms.
The MLSS mixed liquor suspended solids of RAS ranges from 10,000 to 20,000mg/l
(it needs to be high for aeration tank)
MLSS in aeration tank is 2000 - 4000 mg/l.
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The activated sludge process is a continuous process of growth and decay of
micro-organism. The objective for activated sludge is to retain a mixed microbial
population on a growth curve for optimum performance.

types of activated sludge system include
 Complete mix
 plug flow
 Oxidation ditch
 Contact stabilisation
 Sequencing batch reactor

1a     Complete mix activated sludge
 In a complete mix activated sludge plant, the reactors have uniform characteristics
throughout entire reactor. Aeration can be provided by surface aerators or
submerged bubble diffuser aeration systems.

A low level of food is available to the microbes with operating F/M ranging from 0.04
to 0.07. Volumetric loading is typically < 1 kg BOD5/d/cubic m
Dissolved oxygen levels maintained at > 2 mg/l which may be difficult in large tanks
far from the surface aerator. Generally the RAS from the clarifier is fed directly to
the aeration tank where it is mixed completely with the existing contents.

The advantages of this modified activated sludge system is that it can withstand
shock loads due to low F/M ratio and there is good flexibility in utilising wide range of
loads. It is a particularly good system for treating high organic industrial
wastewater.

1b     Plug flow reactors
These refer to systems where a plug of substrate influent to an aeration basin is
moved forward without too much interaction with the plug before or after it. The
RAS is returned to mix with influent. Aeration occurs at front end and this means
that mixing occurs in lateral direction but not longitudinal.

In these systems there is a BOD concentration gradient from inlet to outlet i.e. high
organic loading at influent end of basin and excess food substrate at influent
corresponding to log growth phase and high F/M ratio. At the outlet end, there is a
shortage of food substrate and the micro-organisms are in the endogenous phase
(bacteria consuming dead cells and using own stored energy).

The advantages of this system is that it is able to treat fully all influent and the
influent typically stays in aeration basins for longer periods than complete mix
systems.

1c     Oxidation ditch
The aeration basin or oxidation ditch is usually a racetrack configuration with cage or
brush aerators at one or more locations. Influent enters upstream of the aerator and
moves forward as a plug from location of high oxygen level to low oxygen levels.
The RAS is recycled to influent. This system is characterised by long Hydraulic
Retention Times (HRT) (about 24 h)
and long MEAN CELL RESIDENCE TIMES (20 to 30 days).
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The advantages of this system is that it is suited where both BOD and nitrogen
removal desired.

1d     Oxygen activated sludge
This process involves operating activated sludge systems in oxygen atmosphere
rather than air. The oxygen is usually generated on site by cryogenic air
separation or pressure swing adsorption units.

The activated sludge tanks are covered and usually 3 stages are operated in series.
The use of oxygen allows highly aerobic conditions to be established
i.e. dissolved oxygen concentrations in excess of 6 mg/l are maintained in the
treatment tanks. High aerobic conditions allows high MLSS values of
4000-9000mg/l and F/M ratios of 0.6 – 1.0 to be used without stability problems of
sludge. After the final stage, Oxygen is depleted to around 50% of the headspace
and the gas is vented into the atmosphere

ADV: suited to high organic strength wastewaters         e.g. pulp and paper mill and
organic chemical industries.

1e     Powdered activated carbon (PAC)
Activated sludge systems are dosed with powdered activated carbon to facilitate
removal of organic compounds during biological treatment (PAC doses range from
20 to 200 mg/l). This can help to protect biological processes from inhibitory or toxic
chemicals.

Biodegradation of absorbed chemicals can occur because the residence time in the
system is increased from the hydraulic residence time to the sludge age i.e.
typically from 6 hrs HRT to 5 days (sludge age)

1f      Contact stabilisation
In this system, aeration is carried out in two phases in two different tanks.

Screened sewage is aerated and mixed with return activated sludge for 30 min in a
contact tank. This allows absorption of organics onto solids. The mixed liquor is
allowed to settle and supernatant discharged. The settled mixed liquor is
transferred to a stabilisation tank and aerated for up to 12 hours for further
stabilisation before returning to the influent and co mbining with influent wastewater
to absorb the next batch.

In the contact tank MLSS about 2000 mg/l, In the aeration tank MLSS up to 20,000
mg/l. Aeration volume requirements are usually 50% lower than conventional plug
flow.

Advantages of the system are that the process requires small aeration tanks, the
high initial F/M loading promotes good settleability of sludge and the systems are
suited for package systems and for expansion of systems where space limited

1g     AbsorptionBio-oxidation (A/B) process

This System is two activated sludge systems in series. Each has its own
sedimentation tank with return sludge mechanism. The process is operated without
primary sedimentation with first stage receiving a high sludge loading rate to promote
absorption of organics. The F/M ratio is 3 – 7 kgBOD/KgMLSSd resulting in
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anaerobic treatment in the first stage.
The second stage is conventional biological oxidation

ADV; the energy requirements for the reduction in BOD in the first stage are low.
Overall loading is high relative to treatment in a single stage activated sludge plant.

1h       Sequencing batch Reactors (SBRs)

The sequencing batch reactor (SBR) Is a complete mix activated system without a
secondary clarifier. It has a single aeration basin, and five different sequences are
followed.

Aeration and clarification are carried out in the same tank
Sequence:
1      fill                  basin fills with influent
2      React                 Basin aerated when 100% full
3      Settle                Basin allowed to settle - sedimentation and clarification
              occur
4      Draw                  Effluent withdrawn from top of tank
5      sludge waste          Sludge wasted from bottom of tank

The duration of the cycle varies form 4 to 48 hours and the F/M ratio varies with
cycle length may range from 0.03 to 0.18.
The main advantage of this system is that there is no need for sludge recycle period.
b       Attached growth systems.

ATTACHED GROWTH (or fixed film) allow a microbial layer to grow on surface of
media while exposed to atmosphere (for oxygen supply). The microbial layer
biodegrade organics to Biomass and by products. These systems used microbial
slime layers to reduce BOD of effluents. Traditionally stones were used but recently
plastic media has been used to encourage growth of microbial layer on media with
high surface area to volume ratios

2a     percolating filters
Traditionally, percolating filters were cylindrical or rectangular boxes containing stone
media. Now, plastic media is used for the treatment of industrial wastewaters.
Tank dimensions range from 1 - 2.5 m deep and 5 - 50 m diameter and ventilation
openings allow updraught of air for aeration.

The hydraulic loading and organic loading vary depending on the type of filter
eg;

Design        Low rate       intermediate   high        super         roughing
              conventional   rate           rate        rate          filter
Media         stone          stone          stone       plastic       plastic/stone
Hydraulic     10 000         40 000         100 000     150 000       1 800 000
loading       - 40 000       - 100 000      - 400 000   - 900 000
(cu.m/d/m2
Organic       1-3            3-6            6 - 12      < 30          >20
loading
kg
BOD5/d/cu m
% BOD         80 - 85        50 - 70        40 - 80     65 - 85       40 - 85
removal
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Recirculation may also occur except for low rate and the roughing filter.

Aerobic Biodegradation occurs at the media/Biomass/air interface. At deeper levels
there may also be anaerobic degradation. A build-up of Biomass can occur and
then be sloughed off.

Maintaining Micro-organism communities is very important especially fac ultative
bacteria which are significant micro-organisms.

ADV: Versatile systems because they can treat low strength to advanced standards
or act as roughing filters to high strength wastes and are capable of treating soluble
organics efficiently so they are suited to many industrial wastewaters eg milk
wastes.

1b     Biotowers
The introduction of plastic media has allowed the adaptation of percolating filters
using deeper beds or towers up to 12 m in height. These are called biofiltration
tower systems.

Wastewater flows down through the plastic media. The microbial film develops over
the surface of the media and aerobically degrades the organic material. Oxygen
supply is enhanced with the use of fans to blow air upwards against the flow of
effluent.

Additional nutrient supplements such as nitrogen, phosphorus or potassium may be
required for microbial growth depending on the wastewater.

ADV: they are particularly good for treating high strength industrial effluents such as
vegetable processing and cannery wastes and in the dairy milk industry.

1c     Rotating biological contactor
A cylinder with its axis horizontal rotates into and out of the semicircular wastewater
holding tank. The rotating cylinder is made of high density plastic.
As the cylinder rotates, Biomass builds up on the surface. Wastewater trickles
down surface of contactor and absorbs oxygen from the air

ADV: Used for both low strength and high strength wastes

ANAEROBIC SYSTEMS

Anaerobic digestion is typically used to stabilise sludges. However, anaerobic
stabilisation is also used for treating wastewaters particularly as a roughing stage.
The number of applications of anaerobic digestion to treat industrial effluents has
grown more rapidly on some European countries than others.

               Industrial applications of anaerobic digestion in Europe.

                        Food effluents      Non-food effluents
                           Distillers             Paper
                         Sugar refiners      Pharmaceuticals
                           Breweries           Chemicals
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                       Milk processing          Textiles
                           Abattoirs
                          Canneries
                       Fruit processing
                          Soft drinks
                      Type of digester         % of total
                             UASB                 41
                       Anaerobic filters          22
                      Contact digester            15
                         Stirred tank
                          no recycle               12
                         other types               10

Anaerobic processes tend to be used for wastewater containing high concentration
of organic material. Anaerobic processes are generally used to reduce organic
content of high strength wastes so that can be treated by aerobic biological
processes. Organic wastes are broken down to methane and carbon dioxide in
anaerobic conditions. However the growth rate of anaerobic bacteria are slow so
long sludge retention times are required to achieve a reasonable rate of treatment.


The advantages of anaerobic treatment processes Is the low energy requirements,
low levels of sludge production so low operating costs and slow growth of organisms
means lower nutrient requirements.

3a     Anaerobic contact process
This process comprises an equalisation tank, digester, degasifier and sedimentation
tank. The system provides for separation and recirculation of seed organisms which
allows process operation at retention periods of 6 to 12 hours.

First: the flow is equalised and heated above 30C then, wastewater is fed into
digester and mixed with returned sludge from sedimentation tank. Retention time in
the digester is approx 12 hours. Dissolved gasses are produced which are stripped
in the vacuum degasifiers. Wastewater is passed through sedimentation tanks for
clarification before discharge. Sludge from sedimentation is returned to the digester
to maintain mixed liquor solids to approx. 1000 mg/l.

ADV: low running costs since no aeration needed, sludge production is low and
methane produced during digestion can be used to preheat wastewater with excess
gas sold as fuel. The treatment is robust with resistance to shock loads and low
nutrient requirements relative to aerobic processes and the system has been used
successfully for meat packing wastewaters.

3b     Upward flow anaerobic sludge blanket (UASB)
Wastewater is directed to bottom of reactor where it must be distributed uniformly.
Wastewater flows up through a Blanket of sludge granules produced by chemical
flocculation or from biological activity. Upward flow of wastewater maintains the
suspension of the blanket in the digestion tank and so does attached bubbles of gas
(methane and CO2) produced by anaerobic digestion.

Typical upward flow rates are 0.6 – 0.9 m/h to keep the blanket in suspension
As wastewater passes through the blanket, solids are retained by filtration and
soluble organics are adsorbed and digested anaerobically. Baffles prevent solids
                                                                                     14


escaping.

ADV Used for treating high strength soluble organic effluent e.g. meat packing
waste and also successfully used for sugar beet wastewater

3c    Land disposal / Reed beds
Capacity of land for treating wastes is still utilised for food processing wastes.
Processes responsible are filtration, adsorption, and biological activity

3e      Anaerobic lagoon
Anaerobic lagoons can be used for roughing treatment of high strength organic
wastewaters such as food, and petrochemical effluents. Suspended solids are
typically reduced by 70% by sedimentation. Complex compounds are hydrolysed
by biological activity and so broken down to soluble organics.

Anaerobic and facultative bacteria activity generates organic acids from solubilised
material.. Then follows methanogenesis by anaerobic bacteria which produces
methane and carbon dioxide. These appear as gas bubbles.
The final stage of methanogenic anaerobic microbial digestion substantially reduces
the BOD.
Other examples of anaerobic biological treatment systems include
Anaerobic filter reactor
Fluidised bed

Resource material
Kiely: Environmental Engineering
Metcalf and Eddy: Wastewater Engineering