Agriculture Reference
In-Depth Information
clogging pipes, pumping systems and screens. They
reduce oxygen transfer and can seriously reduce the
efficiency of aerobic treatment systems. They also lead to
odour problems as accumulated grease or fat becomes
rancid.
pH
is a measure of the acidity or alkalinity of an aque-
ous solution. Pure water has a pH value of 7.0. Biological
wastewater treatment processes work best at a pH value
of 7.0 (neutral).
Total nitrogen
(N) occurs in three forms in effluents:
organic nitrogen, inorganic nitrogen and free ammonia
and nitrites.
Ammonia
in solution is toxic to aquatic life;
the maximum discharge to sewers is 40 mg/l. High
nitrate concentrations in natural waters encourage algae
and other plant growth, thus blocking water courses. The
maximum level permissible in potable water is 0.5 mg/l.
Levels of 0.5 mg/l or greater in chlorinated water lead to
taste and odour complaints and also to the development
of THMs or trihalomethanes which are bio-cumulative
and carcinogenic.
Pathogenic bacteria:
Potable water should not contain
any coliform organisms.
Suspended solids
(SS) refer to matter which is insoluble
and is suspended in the water. It consists of both organic
and inorganic components. The organic material will
eventually be degraded.
Temperature
should not be more than a few degrees
above the temperature of the receiving water in order not
to disturb the natural biocycle.
Turbidity and colour:
Effluent should be clear and
colourless.
Volatile solids
(VS) are used as a measure of biogas
production (not applicable to aeration tanks, only useful
in the context of anaerobic digestion).
This can be achieved by the use of grilles over drains,
fat traps, grit settlement and other preliminary treat-
ments, along with continuous dry cleaning or 'clean as
you go' during the operation of the plant and at breaks in
production.
Pollution parameters
Biochemical oxygen demand
(BOD) is a measure of the
readily biodegradable material in a wastewater. It is
obtained by measuring the oxygen consumed by aerobic
organisms, when a known volume of the effluent is added
to a known volume of oxygen-saturated water and incu-
bated at 20°C for 5 days. It is generally used to express the
concentration of pollutant within a wastewater.
Chemical oxygen demand
(COD) is a measure of the
oxygen required for the oxidation of all oxidisable
organic and inorganic matter in a known volume of
effluent, using a standard technique. The COD is often
used as a faster and more accurate means of determining
the oxygen requirements of an effluent before treatment.
A ratio between BOD and COD can be established for a
particular wastewater. COD measurement can be used as
a predictive measurement of BOD.
BOD and COD values vary greatly in the various food
processing operations (Table 3.4). BOD can cause oxy-
gen depletion to waterways into which it is discharged,
whereas COD may not but may contain inhibitors or
toxins that could also have detrimental effect on flora
and fauna.
Chloride
(Cl) is a measure of salinity.
Dry matter
(DM) or
total solids
(TS) is the final weight
of solids derived from a known amount of effluent that
has been dried to a constant weight at 105°C over
24 hours. It is measured in gram per litre or milligram
per litre.
Grease, fat and oil
are a group of substances having
common properties of immiscibility with water and a
lower specific gravity, which cause them to float.
Concentrations are measured by the amount of solvent
required for the effluent to become soluble. Some water
authorities in the United Kingdom will accept a level of
100 mg/l. The substances tend to coat treatment systems,
Treatment
Chemical and microbial standards are set for the
discharge of effluent, and these will depend on the
volume and strength of the effluent and dilution proper-
ties of the receiving watercourse or public sewer to which
it is being discharged.
Air pollution with regard to odours is measured
subjectively and is related to the effect the odour has on
the public, that is, the degree of 'nuisance.
Table 3.4
Average BOD values for some food processing
operations
Preliminary treatment: Screening, solids and
grit removal
Preliminary treatment is based on the removal of solids,
and this is best done by letting all water pass through one
or more screens. These screens should be non-clogging
and self-cleaning, adaptable to variable water flows, easy
to clean automatically or manually (when required),
odourless and noiseless. The resultant screenings may
Source
BOD (mg/l)
Poultry meat plant
1000-1200
Pig meat plant
1500-2000
Cattle/sheep meat plant
1400-3200
Fish processing
1000-3000
Dairy (washings)
600-1300
