Tannery wastewater treatment plant
Tannery wastewaters are one of the most complex cases of treatment and disposal of industrial effluents.
Raw materials are organic, as those coming from food industries, but with a main difference regarding chemical process in with both organic and inorganic reagents are used.
The effluent, as for food industries, requires a large amounts of oxygen to reduce the organic matter, but contains chemical reagents which are toxic or harmful for biological reactions. In case of high concentration, all chemicals are able to inhibit any reactions.
In order to discharge into public sewer or receiving waters, a specific and tailored wastewater treatment plant must be carefully designed, considering the type of tanning method adopted, that can be vegetable or chemical (by the use of chrome).
The main steps of the production cycle can be summarized as follows: soaking, liming, deliming, pickling (pickle), tanning, dyeing and fattening, washing.
Tannery wastewaters composition depends on the tanning process: vegetable or by means of chrome.
Effluent is composed by discharges from several processes and from cleaning operation.
The pollutant load is normally evaluated from the concentrations of some parameters: COD, sulfates, chlorides, chromium, ammoniacal nitrogen and sulphides.
The COD could have very high values, up to 10000 mg/l but, depending on the treatment used, can be reduced up to 90%.
Effluent coming from the vegetable process has a high BOD value and, therefore, a very high oxygen demand. That one coming from the chrome tanning has a high trivalent chromium concentration.
The typical water consumption ranges from 40 up to 90 litres per quintal of fresh leather processed.
Most part of the used water (up to 50%) is lost by evaporation.
During the first production step, the produced effluent contains also salts (chloride) used to preserve hides and to block putrefactive phenomena during the transportation of raw material from production area (slaughterhouses) to tanneries.
The effluent coming from the different working phases is characterised by variable pH values: from very alkaline to very acidic with values ranging from 3-4 up to 11-13
Waste management is currently a serious problem that has no easy solution for the tanning industry.
Tannery wastewaters do not have a high biodegradability degree and need, to be treated by a standard biological treatment plant, an high mixing ratio with domestic effluent. Before this mixing, chlorine and sulphides must be removed and PH correction must be adopted.
Very often, since this kind of industries are usually built far from civilian settlements, the mixing with domestic effluent is not possible, In these cases a targeted treatment is strictly necessary.
The main objective of the clarification phase is the removal of all materials not already removed in the pretreatment, such as suspended solids and various constituents like fats, waxes, mineral oils, floating non-fatty materials, etc.
Sedimentation reduces suspended solids and the BOD.
Acid and alkaline effluent are collected separately in different mechanical pre-treatment lines and then they are mixed together. Acid water is used as neutralizing reagent and coagulant for alkaline water.
Nevertheless the final pH is still alkaline and requires reagent dosing.
Downstream to mechanical pre-treatment, neutralization and chemical dosing, the most common used process is a biological secondary treatment. Usually only aerobic systems are used, due to characteristics of tannery effluents (primarily to their sulphide/sulphate content), wile anaerobic treatment is used in sludge digestion.
The most widely used method is the activated sludge treatment with extended aeration; other biological systems, such as the membrane bioreator (MBR) has not been successful in this area but they may be developed in the future.
Sometimes others treatment solutions can be adopted, as biological filtration or trickling filters, but often they have problems due to calcium build up. In order to avoid this phenomenon it would be necessary to precipitate the calcium carbonate prior to the filtration phase.
Where biological treatment is not enough or where a high performance is required, specific treatments as chemical-physical plant, can get good results. For example a chlorination allows the removal of sulphides.
Anyhow these kinds of plants are very expensive, in terms of purchase and installation and for reagents, considering their constant use for all useful life. For example, the chlorine consumption (Cl) can be estimated from 2 to 8 parts for each hydrogen sulphide (H2S) part to remove.
Coagulation by ferrous sulfate (FeSO4) is not recommended because it creates colloids which are difficult to disperse or dehydrate.
A typical example of physical-chemical treatment plant is made up of four separate discharge lines: pre-tanning, chrome tanning, fattening, dyeing and washing.
The first and the second treatment lines have an equalization and homogenization section, a mixing tank for reagent dosing and a final settlement basin.
A vacuum filter dries the sludge coming from the first line that contains sulphide; a centrifuge dehydrates the sludge coming from the second line that contains chromium hydroxide.
Fattening water is treated with a degreasing and flotation tank.
Treated waters, coming from different lines, are then mixed and stored into the final equalization basin and, after that, discharged into the public sewer.
The reverse osmosis has been applied in different effluent discharging lines and has the important advantage to recover and reuse the concentrate.
A better organization of craftsmanship process and the water recovery should lead to a reduction of the water consumption and, therefore less wastewater to treat and to discharge.
Removed pollutants, for example the chrome, can often be recycled within the production cycle.
Money saved by the reuse of reagents reward investment for the treatment plant.