Pulp and Paper Wastewater Treatment Engineering: Pollution Load, Graded Process Routes and Anaerobic-Aerobic Design Parameters under HJ 2011-2012

1. Status of the standard and two load equations

HJ 2011-2012, Technical specifications for pulp and paper industry wastewater treatment, was approved on 2012-03-19 and took effect on 2012-06-01 as a first issuance. It applies to wastewater treatment projects at pulp and paper mills using chemical, chemo-mechanical, mechanical or recycled-fibre pulping, and serves as a technical basis for environmental impact assessment, feasibility study, design, construction, installation, commissioning, check and acceptance, operation and supervision. It was drafted by the Shandong Provincial Academy of Environmental Science, the Shandong Light Industry Design Institute and the Xinxiang Academy of Environmental Science in Henan.

Clauses 4.1 and 4.2 provide two groups of estimation equations. For flow: Q = Qi + Qj with Qi = the sum of qi mi, where Qi is process wastewater flow and Qj other wastewater such as floor washings and domestic sewage, determined with reference to GB 50015 and GB 50336. For quality: C = (Wi + Wj) x 1000 / Q with Wi = the sum of wi mi, where Wi is the process pollutant load and Wj the load from other wastewater. The unit product drainage qi and unit product pollutant load wi are taken from Annex A.1 and Annexes A.2 and A.3 respectively. Clause 4.1.2 further provides that the maximum daily maximum hourly flow equals the maximum daily average hourly flow multiplied by a peaking factor, which may be taken as 1.1 to 1.4 where no data exist.

2. Read the quality bands first: six wastewater types in Table 1

The typical quality ranges in Table 1 differ enormously and determine the process route directly, in mg/L with pH dimensionless:

• Chemical pulp: pH 5 to 10, SS 250 to 1500, CODCr 1200 to 2500, BOD5 350 to 800, AOX 2 to 26, total nitrogen 4 to 20, ammonia nitrogen 2 to 5, total phosphorus 0.5 to 2;
• Chemo-mechanical pulp: pH 6 to 9, SS 1800 to 3800, CODCr 6000 to 16000, BOD5 1800 to 4000, AOX 0 to 3, total nitrogen 5 to 10, ammonia nitrogen 3 to 5, total phosphorus 1 to 3;
• Mechanical pulp: pH 6 to 9, SS 850 to 2000, CODCr 3200 to 8000, BOD5 1200 to 2800, AOX 0 to 1, total nitrogen 4 to 8, ammonia nitrogen 2 to 5, total phosphorus 0.5 to 1.5;
• Recycled-fibre pulp: pH 6 to 9, SS 800 to 1800, CODCr 1500 to 5000, BOD5 550 to 1500, AOX 0 to 1, total nitrogen 5 to 20, ammonia nitrogen 4 to 15, total phosphorus 0.5 to 1;
• De-inked recycled pulp: pH 6 to 9, SS 450 to 3000, CODCr 1200 to 6500, BOD5 350 to 2000, AOX 0 to 1, total nitrogen 3 to 10, ammonia nitrogen 2 to 6, total phosphorus 0.5 to 1.5;
• Papermaking wastewater: pH 6 to 9, SS 250 to 1300, CODCr 500 to 1800, BOD5 180 to 800, AOX 0 to 1, total nitrogen 2 to 4, ammonia nitrogen 1 to 3, total phosphorus 0.5 to 1.

Table 1 carries three important notes. Wood pulp takes the middle to low values and non-wood pulp the high values. Domestic small paper machines take the middle to low values and imported machines the high values. Most notably, ammonia-base chemical pulp wastewater carries ammonia nitrogen of 55 to 150 mg/L and total nitrogen of 60 to 160 mg/L, far above comparable categories. Clause 4.2.3 adds that where treated wastewater is reused in production, the cumulative effect of pollutants in the reuse water must be considered.

3. Process route: graded by discharge destination

Clause 6.3.3 fixes the correspondence between treatment depth and discharge standard. Pulp and integrated mills meeting Tables 1 and 2 of GB 3544 may choose primary plus secondary, or primary plus secondary plus tertiary. Mills meeting Table 3 shall choose primary plus secondary plus tertiary. Paper mills meeting GB 3544 should choose primary plus secondary. Clause 6.3.2 requires load reduction at source first: raw-material preparation wastewater is pretreated and reused in preparation, with the remainder going to the combined system; where no anaerobic unit is installed, mechanical and chemo-mechanical pulp wastewater should be pretreated before mixing for aerobic and tertiary treatment, whereas with an anaerobic unit in place it may be mixed directly.

Chapter 6.2 sets out the source reduction paths. Black liquor from alkaline chemical pulping shall be treated by combustion-based chemical recovery, with storage and equalisation for concentrated and hot liquids, reuse of clean condensate, and lime mud disposal: grass-pulp lime mud should be used comprehensively, for example to produce refined calcium carbonate or for wet flue-gas desulphurisation of in-house boilers, while wood-pulp lime mud should be regenerated to lime. Ammonium sulphite spent liquor should be evaporated, concentrated and dried to recover lignosulphonate. The washing, screening and bleaching section should use multi-stage counter-current washing and graded counter-current reuse.

4. Primary treatment: from screens to equalisation

• Raw-material preparation wastewater (clause 6.4.1): screen and mesh filtration first to remove coarse solids, then sedimentation or coagulation sedimentation; bagasse-based preparation wastewater may also be treated anaerobically. Plain sedimentation uses a surface loading of 0.8 to 1.2 m3/(m2 h) and hydraulic retention of 2 to 4 h; coagulation sedimentation uses a mixing zone G of 300 to 600 per second for 30 to 60 s, a flocculation zone G of 30 to 60 per second for 5 to 10 min, and a separation zone surface loading of 1.0 to 1.5 m3/(m2 h) with retention of 2 to 3 h.
• Screens (clause 6.4.3.2): coarse screens should be mechanically cleaned with 10 to 20 mm openings and 0.6 to 1.0 m/s velocity through the openings; fine screens should be self-cleaning mechanical units with 2 to 5 mm openings; the working platform above the screen channel shall stand 0.5 m above the highest design water level ahead of the screen.
• Collection tank (clause 6.4.3.3): hydraulic retention time should be 10 to 30 min; a sump shall be provided with a floor slope of not less than 0.01; an emergency overflow, flushing and sludge removal devices, and level control with alarm are recommended.
• Fibre recovery (clause 6.4.3.4): unpowered curved fine screens with slot widths of 0.2 to 0.25 mm; gravity self-flow screens of 60 to 100 mesh with a hydraulic capacity of 10 to 15 m3/(m2 h); rotary filters and mechanical drum fine screens with slots of about 0.2 mm.
• Primary sedimentation (clause 6.4.3.5 b): surface loading of 0.8 to 1.2 m3/(m2 h) and hydraulic retention time of 2.5 to 4.0 h; surplus sludge from the final clarifier may be returned to the primary tank.
• Coagulation sedimentation (clause 6.4.3.5 c): mixing zone G of 300 to 600 per second for 30 to 120 s; flocculation zone G of 30 to 60 per second for 5 to 20 min; separation zone surface loading 1.0 to 1.5 m3/(m2 h) and retention 2.0 to 3.5 h.
• Conventional flotation (clause 6.4.3.5 d): air-water contact time of 30 to 100 s, surface loading of 5 to 8 m3/(m2 h) and hydraulic retention of 20 to 35 min.
• Equalisation tank (clause 6.4.3.6): the effective volume shall hold more than 4 h of the maximum daily average hourly flow; with submersible mixers the power density should be 4 to 8 W/m3, and with aeration the air rate should be not less than 4 m3/(m2 h); nutrient dosing and pH adjustment ahead of the tank are recommended.

5. Secondary treatment: anaerobic and aerobic boundaries

5.1 Three selection thresholds

• Concentration threshold (clause 6.4.4.1): where the CODCr concentration after primary treatment is above 2000 mg/L, anaerobic plus aerobic is preferred; where it is below 1200 mg/L, aerobic treatment is preferred.
• Nutrient ratio threshold (clause 6.4.4.2): wastewater entering the anaerobic system shall reach BOD5:N:P of 200:5:1; for the aerobic system, 100:5:1.
• Temperature threshold (clause 6.4.4.3): the water temperature in the anaerobic reactor shall be held between 25 and 38 degrees C; in the aerobic reactor, between 10 and 35 degrees C.

5.2 Three inhibition boundaries for the anaerobic unit

Clause 6.4.4.4 sets explicit inhibition control values, and this is the most useful part of the specification for engineering purposes: the influent suspended solids concentration to UASB and IC reactors should be held below 500 mg/L; the ratio of sulphate to CODCr in the anaerobic influent should be kept below 10 percent with sulphate below 450 mg/L; where concentrations are higher, a pre-acidification tank should be provided, with the pre-acidification pH at about 6.5 and a hydraulic retention time of about 2 h; hydrogen sulphide generated during pre-acidification should be collected for recovery or treated before discharge. Clause 6.1.3 also recommends that the biological treatment units be designed as two parallel trains.

5.3 Anaerobic design parameters (Table 2)

• Upflow anaerobic sludge blanket (UASB): temperature 32 to 35 degrees C, sludge concentration 10 to 20 g/L, volumetric loading 5 to 8 kgCODCr per cubic metre per day, hydraulic retention time 12 to 20 h, surface loading 0.5 to 1.5 m/h, biogas yield 0.4 to 0.5 cubic metres per kg CODCr;
• Internal circulation reactor (IC): 32 to 35 degrees C, sludge concentration 20 to 40 g/L, volumetric loading 10 to 25 kgCODCr per cubic metre per day, hydraulic retention time 6 to 12 h, surface loading 3 to 8 m/h, biogas yield 0.4 to 0.6 cubic metres per kg CODCr;
• Completely mixed anaerobic reactor (CSTR): 30 to 38 degrees C, sludge concentration 5 to 8 g/L, volumetric loading 3 to 6 kgCODCr per cubic metre per day, hydraulic retention time 18 to 28 h, sludge return ratio 100 to 150 percent, biogas yield 0.4 to 0.5 cubic metres per kg CODCr.

Three dimensional constraints also apply (clauses 6.4.4.4 e to h). UASB and IC units shall be provided with uniform distribution devices and three-phase separators; effluent from the separation zone shall discharge over an overflow weir with a scum baffle ahead of it. External recirculation may be used to raise the upflow velocity, with the recirculation rate automatically adjusted according to the set reactor surface loading and biogas production. The effective height of a UASB is generally 5 to 7 m and should not exceed 10 m, with a single unit volume not exceeding 2000 m3; the IC height should not exceed 25 m with a single unit volume not exceeding 1500 m3. A sedimentation tank shall follow the CSTR, with a surface loading of 0.6 to 0.8 m3/(m2 d) and a settling time of 4.0 to 6.0 h; where a lamella or tube settler is used the surface loading may be raised appropriately.

6. Aerobic stage and tertiary treatment

Clause 6.4.4.5 recommends extended-aeration processes with low organic loading and high shock resistance for the aerobic unit, such as oxidation ditches, completely mixed aeration with a selector, SBR and two-stage aerobic treatment; where ammonium sulphite pulping wastewater is treated, an anoxic/aerobic (A/O) process with nitrogen removal shall be used. Table 3 gives the main parameters. Oxidation ditch: MLSS 3.0 to 6.0 g/L, sludge loading 0.1 to 0.3 kgCODCr per kg MLSS per day, volumetric loading 0.4 to 1.2 kgCODCr per cubic metre per day, HRT 18 to 32 h, return ratio 60 to 120 percent, settled sludge ratio 50 to 80 percent, sludge age 18 to 25 d. Completely mixed aeration: 2.5 to 6.0, 0.15 to 0.4, 0.5 to 1.5, 15 to 30 h, 100 to 150 percent, 30 to 80 percent, 12 to 20 d. A/O: 2.5 to 6.0, 0.15 to 0.3, 0.5 to 1.2, 15 to 32 h, 80 to 150 percent, 30 to 80 percent, 15 to 25 d.

The oxygen demand estimate is the practical handle in this clause: the calculation shall follow GB 50014, or may be estimated at 0.6 to 1.2 kg O2 per kg CODCr, with the oxygen demand for ammonia removal added where ammonium sulphite pulping wastewater is treated. The choice of aeration method reflects the specific character of pulp and paper wastewater: full account shall be taken of its tendency to scale, and anti-clogging, easily maintained options such as surface aerators, brush aerators, disc aerators and jet aerators are preferred. Aeration tanks shall also be provided with foam barriers and control measures.

Tertiary treatment (clause 6.4.5) progresses through coagulation sedimentation or flotation, filtration and advanced oxidation. Coagulation sedimentation uses a surface loading of 0.8 to 1.5 m3/(m2 h) with HRT of 2.5 to 4 h, and where lamella or tube settlers are used the surface loading may be taken at 1 to 2 times that of a conventional tank. Conventional flotation uses a contact time of 30 to 100 s, surface loading of 6 to 9 m3/(m2 h) and HRT of 20 to 30 min; shallow air flotation uses an effective depth of 500 to 700 mm with a tank HRT of about 3 to 5 min. Where the SS requirement is tight, filtration may be added in series, with the influent SS preferably below 30 mg/L and media chosen from anthracite, quartz sand, ceramsite, polystyrene foam beads, emery, fibre balls and fibre bundles. If the target is still not met, Fenton oxidation applies: oxidation reaction time of 30 to 40 min at pH 3 to 4, followed by neutralisation with NaOH or Ca(OH)2 for more than 10 min to reach pH 6 to 7, with PAM dosed to strengthen coagulation; a Fenton fluidised bed or the return of coagulation sludge may reduce ferrous sulphate dosing.

7. Three requirements that are easy to miss

First, the emergency tank (clause 6.4.6): its effective volume shall accommodate the total volume of the largest single accidental discharge; a lift pump shall be installed to return the wastewater uniformly to the primary sedimentation tank or equalisation tank; a sump shall be provided with a floor slope of not less than 0.01; and a mixing device is recommended. Second, the segmented design flows (clause 5.2.2): screen channels, collection wells and fibre recovery rooms upstream of the equalisation tank are sized at the maximum daily maximum hourly flow, whereas the equalisation tank and downstream biological tanks and final clarifiers are sized at the maximum daily average hourly flow, with sludge treatment and disposal sized on the maximum daily average hourly sludge quantity. One plant therefore carries two design flow bases, and using the wrong one inflates or shrinks the investment directly. Third, explosion protection and fire safety: clause 6.3.1 requires on-line hydrogen sulphide and combustible gas alarms at the collection tank with electrical and related equipment designed for explosion protection; clause 9.2.4 assigns the works to fire hazard Class D or E, except that where an anaerobic unit is included, its fire hazard classification is Class A and it shall be designed to Grade I fire resistance.

On the operating side, clause 12.4.1 sets out the anaerobic operating points: adjust nutrient dosing according to influent characteristics and surplus sludge discharge; control the reactor temperature at 35 plus or minus 2 degrees C using temperature regulation facilities; control VFA and alkalinity by adjusting system load and dosing acid or alkali, holding the mixed liquor pH between 6.8 and 7.6; and improve distribution so as to raise the sludge concentration as far as possible while keeping effluent SS, preferably below 200 mg/L, under control. Clause 12.4.2 gives the aerobic dissolved oxygen targets: below 0.3 mg/L at 0.5 m below the surface in the anoxic zone and below 0.2 mg/L at 1.0 m, with at least 2.0 mg/L at the outlet of the aerobic zone; microscopic examination of the activated sludge shall also be strengthened so as to prevent sludge bulking. Clause 12.3.2 requires on-line monitoring of key indicators such as CODCr and ammonia nitrogen, linked to the supervision centre, and manual sampling at intervals even where an on-line system is installed.

Two closing caveats. Biogas purification, storage and use follow NY/T 1222 and NY/T 1220.2; this specification only indicates that biogas is preferably used as boiler fuel (clauses 6.7.1 to 6.7.3), so the heat balance and hydrogen sulphide removal parameters must be obtained from the dedicated standards. Annexes A and B are informative, so the unit product drainage, pollutant generation and typical process removal efficiencies they contain are reference material; formal design shall be based on measured or comparable data. This article is based on the issued draft; the formally published standard text prevails.