the salt laden tannery effluent from the soaking process can be collected in solar evaporations pans, possibly pretreated with coagulant, after which salt can be recovered. In case of chrome tanning effluents, the wastewater that contains chromium should not be allowed to become mixed with other types of wastewater: it must be collected separately. Depending on the quality of the composite effluents, neutralising chemicals like lime alum, ferric chloride etc. should be added for an effective precipitation of chromium and removal of suspended solids in the sedimentation process. From this material chrome can be recovered, or dumped separately [43]. Treatment. This involves separation of solids in a settling tank (primary clarifier), or by flotation. The settleable solids and up to 60% of the suspended solids corresponding to approximately 35% of the BOD, can be eliminated during the primary treatment. Subsequently the solids may be treated by anaerobic sludge digestion. This produces biogas and solids that are suitable for soil conditioning and fertilization. Primary treatment is a essential activity that needs to be undertaken for a proper application of various secondary treatment systems. In case of aerobic secondary treatment, a further function of this step is the reduction of electric energy required for aeration [11]. Treatment. This usually consists of biological treatments by means of high rate anaerobic treatment systems, anaerobic (lagoons) suitable for high organic loads, or aerobic (lagoons) suitable for low organic loads, activated sludge, oxidation ditch or a combination. Present research is mainly focused on low energy demand and low volume treatment systems and optimum process control. Usually, a combination of high rate anaerobic treatment and aerobic activated sludge is required to meet effluent quality demands. Removal efficiencies reached with these kinds of combination are up to 98-99%. Depending on the operational conditions, removal efficiencies for slaughterhouses range from 70 to more than 99% for BOD and grease and from 80 to more than 97% for Suspended Solids (SS). The process performance depends strongly on the amounts of SS that can be removed in the primary treatment phase [38]. Treatment. This includes chemical-physical methods such as adsorption, stripping, coagulation, sedimentation, chlorination as well as biological methods like slow sand filtration and maturation ponds. This post-treatment serve to remove nutrients such as phosphorus, sulphide, suspended solids, remaining BOD as well as pathogens [1]. Method of wastewater treatment is that of irrigation on land. Before wastewater is applied on land, toxic compounds such as chromium, salt sulphide, etc. have to be removed. Small amounts of nitrate and phosphate however may serve as fertilizers. The BOD value is usually not allowed to be higher than 300 mg/l.present, this kind of wastewater treatment is carried out mainly in developing count...
