Animal by-product installation waste water treatment .1 Rendering waste water treatment

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RHC/EIPPCB/SA_BREF_FINAL Version November 2003 99 treatment. The process does, however, reduce the pathogenic content of the waste water [216, Metcalf and Eddy, 1991].

Biosolids produced by the treatment plant may, e.g. be dewatered prior to land spreading as a soil conditioner or digested to yield biogas. Limitations on land spreading and land injection are leading to an increased trend towards the incineration of sludges [244, Germany, 2002]. Sludge storage, handling and spreading can lead to odour problems. As well as managing the usual operational issues related to activated sludge plants, such as the development of bulking sludges or carrying excessive biomass inventories, particular problems may be experienced with slaughterhouse effluents, which can cause the formation of biologically stable foam; or they may contain biocidal substances capable of inhibiting microbial activity. [12, WS Atkins-EA, 2000]

Removal of nitrogen and phosphorus

Processes have been developed which combine the carbon oxidation, nitrification and dentrification steps in a single process. These processes have several advantages, including reducing the volume of air needed to achieve nitrification and BOD removal; elimination of the need for the addition of organic sources to provide carbon for denitrification and elimination of the need for intermediate clarifiers and return-sludge systems required in a staged nitrification system. It has been reported that most of the systems can remove 60 - 80 % of the total nitrogen, although removal rates of 85 - 95 % have also been reported.

In the combined processes, the carbon in both the waste water and in the micro-organisms after endogenous respiration during the aerobic treatment, is used to achieve denitrification. For denitrification, a series of alternating aerobic and anoxic stages, without intermediate settling, have been used. Anoxic zones can be created, e.g. in oxidation ditches, by controlling the oxygenation levels. The sequencing batch reactor is also suited to providing aerobic and anoxic periods during the operating cycle and can achieve a combination of carbon oxidation, nitrogen reduction and phosphorus removal. Phosphorus can be removed by coagulant addition or biologically without coagulant addition. If the sequence: fill, anaerobic, aerobic, anoxic, settlement and decant is followed then phosphorus release and BOD uptake will occur in the anaerobic stir phase, with a subsequent phosphorus uptake in the aerobic stir phase. By modifying the reaction times nitrification or nitrogen removal can also be achieved. Carbon from the endogenous respiration phase can be used in the anoxic phase to support denitrification. [216, Metcalf and Eddy, 1991]

2.3.1.3 Tertiary slaughterhouse waste water treatment

Tertiary treatments, such as filtration, e.g. using sand filters, reed beds, coagulation, or precipitation, are sometimes used as a final cleaning step for the treated effluent, to reduce the BOD and suspended solids, prior to discharge to a water course.

2.3.2 Animal by-product installation waste water treatment

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100 Version November 2003 RHC/EIPPCB/SA_BREF_FINAL 2.3.2.1.1 Mechanical waste water treatment

The mechanical stages in the waste water treatment are implemented before any mixing or equalisation takes place. In the rendering industry, sludge catchers, fat separators, sieves, micro-strainers and settlement tanks are normally used. Undissolved animal matter, such as fat and fat particles, meat residues, hair, bristles and mineral admixtures from the process water can be conveyed back into the production process. Fat separation can be difficult, as the animal fat in the waste water can exist in a very fine form. This is especially true if the water temperatures are high and when the waste water contains tensides. High pH values also impair the fat separation, due to saponification.

Fat separators situated before the mixing and equalisation tanks need to be dimensioned for the maximum waste water production foreseeable. This maximum production occurs, for instance, during the exhaust vapour relaxation. Other issues such as temperature, the influence of rinsing and cleaning agents and the production of the different types and densities of fat need to be considered at the design stage.

In many plants, the fat separator is followed by additional strainer units with sieve apertures of 0.5 - 2 mm, for even more extensive solids separation.

2.3.2.1.2 Physico-chemical treatment

Physico-chemical methods, particularly flotation methods, are used for an extensive separation of the fat and solids. Flocculation agents are used as and when required. Fat retention can be done in fat traps with manual or automatic cleaning. If the fat is emulsified or contains stickwater from the edible fat rendering department, the separation can be very difficult. In such cases it is necessary to use chemical precipitation and flotation.

As with the fat separators, the efficiency degree of flotation plants is reduced by high temperatures and high pH values. Mechanical flotation using air supplied from especially developed submerged flotation aerators, is the method least susceptible to high pH values.

Stripping can be used for the treatment of the hot exhaust vapour condensates (EVCs). Due to low waste water volume flows, it can also be used for the main waste water stream. It is reported that any neutralisation of the stripping effluent is not done immediately after the stripping, but only after the reconvergence with the other waste water part-streams.

The position of the stripping plant in the waste water treatment process is shown in Figure 2.24.

Fat separator

Drum

sieve ME Flotation

Neutrali-sation

Alkalini-sation

Ammonia Stripping EVC

Further WW

AB

- EVC Exhaust vapour condensates - ME Mixing and equalisation tank - further WW Further waste water - AB Anaerobic biology

Figure 2.24: Block diagram of a mechanical/physical-chemical preliminary waste water plant [163, German TWG Members, 2001]

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RHC/EIPPCB/SA_BREF_FINAL Version November 2003 101 Another option to reduce the ammoniacal nitrogen is ammonia conversion. The ammonia is conveyed with the exhaust vapours into a washing tower (converter) countercurrent to a nitrous solution and ammonium nitrate is formed. The ammonium nitrate is extracted from the filter tower when the desired concentration has been reached. The exhaust vapours freed from the ammonia are then condensed into acid exhaust vapours.

For the operation of such a converter, it is necessary that the exhaust vapours do not carry any solids. Cyclones or other suitable separation implements must, therefore, be installed before the converters.

2.3.2.1.3 Biological treatment

Aerobic part-treatment can be used to remove some of the organic material and thereby reduce the BOD of the waste water. It is sometimes undertaken at installations, prior to further waste water treatment at a municipal WWTP.

The composition of waste water from the rendering industry makes it suitable for anaerobic pretreatment. It is, however, unsuitable for the total elimination of the organic load or the elimination of nitrogen. The presence of sulphides may also cause problems.

Anaerobic treatment is generally followed by aerobic treatment to remove nitrogen (and phosphorus) at the rendering site or at the municipal WWTP. Release of phosphorus occurs under anoxic conditions. Thus, biological phosphorus removal requires both anaerobic and aerobic reactors or zones within a reactor. [216, Metcalf and Eddy, 1991]. Anaerobic pretreatment of the waste water is suitable, especially for indirect discharge combined, with a physico-chemical nitrogen elimination.

2.3.2.1.4 Feathers - elimination of hydrogen sulphide

For waste water with high sulphide concentrations, for instance the part-streams from the feathers processing, another preliminary treatment target is the reduction of the H₂S concentrations. Contents from about 80 – 100 mg/l of sulphide will impair the activated sludge biocoenosis and thus the subsequent biological treatment process.

Hydrogen peroxide may be used for the treatment of sulphide-containing waste water. To oxidise 1 kg sulphide stoichiometrically, about 13 litres of 30 % hydrogen peroxide are needed.

The reaction time is about 10 minutes. [163, German TWG Members, 2001]

2.3.2.2 Fish-meal and fish-oil manufacturing waste water treatment

One plant has reported that it uses DAF on-site and then discharges its waste water to a local municipal WWTP.

2.3.2.3 Blood processing waste water treatment

A propriety WWTP has been described for a given blood processing plant. The first step is a physico-chemical treatment, during which polyamines and polyelectrolyte flocculants are added. This is followed by decanting the sludge to another tank. The clarified liquid is also transferred to another tank, where it is corrected for pH and anti-foaming agents are added. The liquid is then subject to a series of aerobic and anaerobic treatments. The WWTP is covered to prevent the release of NH3, from the breakdown of protein. The sludge is used in composting, due to its high protein content.

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102 Version November 2003 RHC/EIPPCB/SA_BREF_FINAL 2.3.2.4 Gelatine manufacture waste water treatment

The water from the bone washing is cloudy and contains particles such as bone fragments, which are removed by screens. Solids are removed using sieves made from, e.g. a mechanical screen constructed from wedge wire. The solids are brushed off the screen into a skip, for disposal to landfill.

The liquid, which is highly organically contaminated [244, Germany, 2002], is delivered to a primary and secondary settlement tank, to allow separation of solids. Iron(III)chloride is injected with either H₂SO₄ or NaOH, depending on the pH, along with polyelectrolyte flocculant. The resultant liquor undergoes aerobic digestion using activated sludge. Nitrification and denitrification steps are also required [244, Germany, 2002]. A clarification step may be followed to remove the activated sludge. The resultant sludge is rich in nitrogen, phosphorus and calcium and is used for land injection and land spreading, possibly after mixing with other ingredients. Alternatively, the sludge may be used to produce biogas [349, GME TWG members, 2003].

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