Sulphide, chrome bearing and general effluents are segregated and screened with an automatic mechanical fine screen with 1 mm slot size. The fine screening removes particles, fibres and solids and protects the subsequent treatments preventing blockages of pumps and pipes.
The sulphide oxidation is conducted in 2 sulphide oxidation tanks, with alternating operation in batch to remove sulphides from liming liquors and washes. A blower assisted jetox venturi aeration provides for high oxygen transfer and mixing, resulting in efficient sulphide oxidation, achieving sulphides of <1 mg/l. A sulphide oxidation reduces Hydrogen Sulphide gas emissions and odours of Balancing and sludge de-watering.
Aerated and mixed balancing
Balancing of the combined effluents homogenizes the various streams and provides an uniform effluent at constant flow rate to the next treatment steps. Tannery effluents are pumped with a recirculation pump through self-entrained jetox venturis, achieving efficient mixing and aeration of the Balancing tank and preventing re-formation of sulphides.
Dissolved Air Flotation (DAF)
Dissolved air flotation is the most effective primary treatment for tannery effluents. Air is mixed at high pressure with the effluent generating small air bubbles which carry suspended solids to the surface, where sludge is removed with a scarper. This allows removing efficiently tiny fibres, which do not settle, as well as fats. DAF treatments achieve an effluent with less than 50mg/l suspended solids and high reductions of COD. A DAF unit is compact and fully automated and includes a coagulant and polymer mixing units as well as automatic sludge drainage.
After primary treatment soluble COD, BOD and ammonia remain in the effluent, which can be removed efficiently by the means of biological activated sludge treatment. Conventional biological treatment comprises of an anoxic pre-denitrification and an aerobic biological tank with a biological clarifier for solid-liquid separation. A conventional biological system achieves about 90% COD and 95% BOD reduction with less than 30 mg/l of suspended solids in the clarified effluent.
Membrane bioreactor treatment
Membrane bioreactor treatment (MBR) is the combination of a biological treatment with ultrafiltration membranes. The MBR treatment mineralises completely all organic pollutants, which goes along with a significant reduction of surplus sludge production and an improved effluent quality with reduction of 92% COD, 99 % BOD and less than 5 mg/l of suspended solids in the effluent.
Nanofiltration and reverse osmosis for process water recycling
The effluent after MBR treatment can be further treated with nanofiltration / reverse osmosis (NF/RO) achieving up to 70% process water recycling of high quality soft water. The concentrate of the NF can be concentrated further with RO to minimise the volume of post concentrate treatment.
Primary and biological sludge can be de-watered with a decanter centrifuge or a screw-, filter-, or belt-press achieving a filter cake of 25-40% dry matter. The volume of filter cake can be further reduced by operating a sludge dryer. The paddle sludge dryer uses macerated steam heated screws, which convey the sludge through the dryer, achieving a reduction of disposable sludge volumes and costs to about 1/3 and increasing the filter cake dryness to 90%.