Tannery and Fur Processing Wastewater: Segregated Pretreatment and Anaerobic Limits in HJ 2003-2010

1. The pollution generation profile dictates segregated pretreatment

HJ 2003-2010, Technical specifications for wastewater treatment of leather and fur processing industry, was approved by the former Ministry of Environmental Protection on 2010-12-17 and took effect on 2011-03-01. Annex B, an informative annex, gives the pollution generation profile that is the starting point for the whole process route. Table B.1 gives pollutant generation per tonne of raw hide: for tannery wastewater CODCr 150 to 250, suspended solids 100 to 150, BOD5 70 to 110, ammonia nitrogen 15 to 30, total nitrogen 20 to 40, total chromium 2 to 5, sulfide 3 to 10, sulfate 30 to 70 and animal and vegetable oils 20 to 100 kg per tonne; for fur processing wastewater CODCr 70 to 150, suspended solids 50 to 80, BOD5 35 to 60, ammonia nitrogen 2 to 5, total nitrogen 7 to 12, total chromium 1 to 4, sulfate 15 to 20 and oils 15 to 65 kg per tonne.

Table B.2 is even more useful, giving the contribution of each process step: the liming step accounts for 92 to 97 percent of the sulfide load and 50 to 55 percent of CODCr; deliming and bating account for 65 to 75 percent of ammonia nitrogen; and chromium tanning accounts for 70 to 80 percent of total chromium and 55 to 60 percent of sulfate. Three characteristic pollutants - sulfide, ammonia nitrogen and chromium - are therefore highly concentrated in single process steps, which gives a direct basis for segregated collection and separate pretreatment. This is exactly why clause 6.2.1 requires that chromium containing wastewater be pretreated to meet the limit before mixing with other streams, and that sulfide containing and degreasing wastewater be pretreated.

Clause 6.2.4 splits the process combinations into three tiers by discharge destination: enterprises discharging to a centralised tanning park treatment plant should adopt pretreatment, or pretreatment plus primary treatment; those discharging to a municipal wastewater treatment plant should adopt pretreatment plus primary, or pretreatment plus primary plus secondary; and those discharging directly to natural water bodies should adopt pretreatment plus primary plus secondary, or that plus advanced treatment. This clause is the direct basis for project scale and investment estimates.

2. Chromium pretreatment: the six steps of alkaline precipitation and recovery

Clause 6.3.1.1 requires recycling or alkaline precipitation combined with the production process, with the treated stream allowed into the combined wastewater works only after its chromium content meets the limit. Clause 6.3.1.2 gives the full parameter chain for alkaline precipitation:

• Precipitants include MgO, NaOH, Ca(OH)2, Na2CO3 and NaAlO2, with MgO and NaOH preferred; the dose is calculated from chemical equilibrium and the chromium liquor pH is controlled at 8.5 to 10.0;
• The storage tank retention is preferably above 2 days, the precipitation reaction time is preferably 1 to 2 hours and the settling time shall exceed 3 hours;
• The chromium sludge is preferably dewatered on a plate and frame filter press, with a cycle of 4 to 6 hours and a throughput of about 1.5 kg per square metre per cycle;
• Acidification uses mechanical or air agitation, with a reaction pH preferably of 2.0 to 2.3, a reaction time above 1 hour and a settling time of 3 to 4 hours; the sulfuric acid dose follows equation (5), M equals 1.93 A V divided by B, where A is the Cr2O3 content of the spent chromium liquor in grams per litre, V is the liquor volume in litres and B is the mass fraction of industrial sulfuric acid;
• The recovered chromium liquor is preferably aged before use, with an ageing time of 5 to 7 days and a pH after ageing of 2.5 to 2.8.

The recycling route (clause 6.3.1.3) covers direct recycling and pickling or tanning cycle recycling, comprising screening, storage, purification, chromium replenishment, conditioning and reuse; the text states clearly that control parameters shall be selected after testing in light of the production process and product type. Annex C.1 gives typical alkaline precipitation efficiencies of 70 to 90 percent for suspended solids, 60 to 80 percent for CODCr and above 99 percent for total chromium, while the main text gives a reference range of 0.1 to 1.5 mg/L total chromium in the combined wastewater after chromium pretreatment (with 0.1 to 1.0 mg/L for another operating condition).

3. Sulfide containing wastewater: three routes and their control windows

3.1 Catalytic oxidation (clause 6.3.2.2)

Manganese salt catalytic oxidation is preferred, using manganese sulfate, manganese chloride or potassium permanganate, with manganese sulfate the usual choice at a dose preferably equal to 5 percent of the sulfide content. pH is preferably held between 10.5 and 13.0, reaction temperature between 15 and 40 degrees C, and the catalytic oxidation reaction time above 6 hours. Aeration may be by blower, using coarse or medium bubble diffusers, or mechanical; with blower aeration the oxygen supply shall exceed 1.1 kg per kg of sulfide, and with mechanical aeration the power input should exceed 0.6 kW per kg of sulfide, while also meeting the mixing requirement.

3.2 Chemical coagulation (clause 6.3.2.3)

The route comprises coagulation and sedimentation or flotation, commonly using iron or aluminum salts. With ferrous sulfate, the pH may be adjusted to 8 to 9 with acid before the reaction and the endpoint pH controlled at about 7; the coagulation time is preferably 10 to 15 minutes; the settling time is preferably 3.0 to 5.0 hours with a surface loading of 0.8 to 1.5 m3 per square metre per hour; and with flotation the surface loading is preferably 1.5 to 2.5 m3 per square metre per hour where no other data exist. Annex C.1 gives efficiencies of 60 to 80 percent for suspended solids, 55 to 75 percent for CODCr and above 95 percent for sulfide by chemical coagulation; only 10 to 20 percent for CODCr but above 90 percent for sulfide by catalytic oxidation; and 55 to 80 percent for suspended solids, 55 to 75 percent for CODCr and above 90 percent for sulfide by acidification recovery. This shows that catalytic oxidation functions primarily to remove sulfide rather than to cut COD, and must not be selected as a COD removal unit.

3.3 Acidification and hydrogen sulfide recovery (clause 6.3.2.4)

The route comprises acidification, solid liquid separation and alkaline absorption. The waste liquor in the acidification reactor is adjusted to pH 4.0 to 4.5 with acid, with a reaction time preferably above 6 hours; hydrogen sulfide is preferably drawn continuously by a vacuum pump to the absorption tower, and the absorption system must be kept under negative pressure throughout; the absorption liquor is prepared with NaOH; the acidified liquor is separated to recover protein; and the separated liquor may be regenerated with CaO, with the regenerated liquor adjusted to a pH of about 12.

4. Combined wastewater: inhibitors and parameters for the anaerobic stage

Clause 6.3.1.3 is the first principle of anaerobic design for tannery wastewater: the selection of anaerobic technology shall give full consideration to the inhibition of anaerobic bacteria by sulfide, sulfate, chromium, neutral salts and the low CODCr to total nitrogen ratio, and cleaner production measures shall be strengthened to reduce the concentration of toxic factors. This explains why the specification places sulfide and chromium pretreatment ahead of the anaerobic unit.

4.1 Pretreatment stage (clauses 6.3.4.1 to 6.3.4.4)

• Screens: with mechanical cleaning the coarse screen opening is preferably 10 to 20 mm, and with manual cleaning 15 to 25 mm; the fine screen is preferably a self cleaning rotary mechanical type with an opening of 2 to 5 mm; the working platform above the screen shall stand 0.5 m above the highest design water level upstream;
• Grit or pre sedimentation: the pre sedimentation tank has a retention of preferably 40 to 120 minutes and an effective depth of 2.0 to 3.0 m, with scum or oil skimming facilities;
• Equalisation: the equalisation time is preferably above 20 hours for a standalone tannery or fur processing enterprise and above 16 hours for a centralised park; where the tank doubles as an accident tank, the volume must include the accident discharge capacity determined as 2 hours of maximum hourly flow; where first flush rainwater is collected, 10 to 15 minutes may be used in the absence of data; submersible mixer power is 2 to 8 W per cubic metre, or with medium or coarse bubble aeration the air rate should not be below 3 m3 per square metre per hour; and the floor slope shall be not less than 0.01, with level control and alarms.

4.2 Sedimentation tank parameters (Table 7)

Primary sedimentation: retention 1.5 to 3.0 hours, surface loading 1.0 to 2.0 m3 per square metre per hour, sludge moisture content 97 to 98.5 percent. Coagulation sedimentation before secondary treatment: 2.0 to 3.0 hours, 1.0 to 1.6, and 96 to 98 percent; after secondary treatment: 2.5 to 4.0 hours, 0.8 to 1.2, and 98 to 99.5 percent. Secondary sedimentation after biofilm: 2.0 to 4.0 hours, 0.8 to 1.5, 96 to 98 percent, with a solids loading not above 150 kg per square metre per day; after activated sludge: 3.5 to 5.0 hours, 0.5 to 0.8, 99.0 to 99.4 percent, with a solids loading not above 150 kg per square metre per day. Where inclined plate or tube settlers are used, the surface loading may be taken as one to two times that of a conventional tank.

4.3 Hydrolysis acidification and anaerobic treatment (clause 6.3.4.5)

• Hydrolysis acidification time is preferably 6 to 12 hours; ambient or mesophilic fermentation is preferred with the mixed liquor temperature in the reactor controlled at 25 to 35 degrees C;
• Tannery wastewater anaerobic units should use a two step anaerobic process or co treatment with other wastewater, while fur processing wastewater may use a single step process; in the two step option the acidification stage may use an anaerobic packed bed or anaerobic contact reactor, and the methanogenic stage and the single step option may use UASB;
• Acidification stage: mixed liquor pH kept below 7.5, sulfide volumetric loading preferably 1.5 to 3 kg per cubic metre per day, CODCr volumetric loading preferably 25 to 45 kg per cubic metre per day, and a sludge yield of 2 to 4 percent;
• Sedimentation after the anaerobic contact reactor: surface loading preferably 1.0 to 1.4 m3 per square metre per day, settling time 3.0 to 5.0 hours and a sludge return ratio of 30 to 50 percent - note that the surface loading here is expressed per day, not per hour, and must be converted before comparing with Table 7;
• Methanogenic UASB: CODCr volumetric loading preferably 5 to 15 kg per cubic metre per day with a hydraulic retention time above 12 hours; the biogas produced should be purified, stored in a gas holder and used as fuel, with purification and storage following NY/T 1222 and NY/T 1220.2.

Annex C.2 provides efficiencies that can be used as a check: primary coagulation sedimentation gives 70 to 90 percent for suspended solids, 50 to 70 percent for CODCr and 45 to 65 percent for BOD; secondary anaerobic plus aerobic gives 85 to 90 percent for suspended solids and CODCr, 95 to 98 percent for BOD and 70 to 95 percent for ammonia nitrogen; and advanced coagulation sedimentation gives 50 to 75 percent for suspended solids but only 15 to 30 percent for CODCr.

5. Sludge, safety and items requiring testing

Clause 6.3.6 sets both quantitative and qualitative boundaries for sludge. Sludge comprises pretreatment sludge, physico chemical sludge from combined wastewater treatment and excess sludge. Estimated on a raw hide basis, the dewatered chromium containing pretreatment sludge amounts to 20 to 30 kg per tonne of raw hide as dry solids. Chromium containing sludge may be made into a chrome tanning agent and reused in the tanning process according to production needs, and where it cannot be reused it is disposed of as hazardous waste. Sludge from combined wastewater treatment is disposed of as hazardous waste if identified as hazardous, and as general solid waste if identified as such - identification before classification is the core of this clause.

Two mandatory safety points are easily missed: clause 9.2.4 assigns the fire hazard category of the wastewater treatment works to class D or E, except for the anaerobic unit; where an anaerobic unit is present, that unit is class A and its fire protection rating shall be taken as grade one; and clause 12.5.3 requires an accident storage tank for chromium containing wastewater together with accident prevention measures to eliminate accidental discharge.

There are two classes of item that must be determined by testing. The first is the control parameters of the recycling route (clause 6.3.1.3), which must be determined by testing against the production process and product type. The second is treatment efficiency: clause 6.2.2 states that it shall be determined by testing or by analogy with comparable enterprises, with Annex C serving only as a reference where no data exist. In addition, this specification does not give specific sludge loadings or volumetric loadings for the aerobic stage. Clause 6.3.4.6 only requires a process with low organic loading, strong shock resistance and nitrogen removal capability, requires the tank volume to be calculated from nitrification and denitrification kinetic formulae with full allowance for low winter temperatures, and leaves the detailed parameters to GB 50014 and to CECS 111 and CECS 112.