Deliverable Report of processes specifications anaerobic waste water purification process of mechanical pulping production

Transcription

1 VOCless pulping Waste Waters Deliverable Report of processes specifications anaerobic waste water purification process of mechanical pulping production Case study of anaerobic waste water treatment process in pilot mill in Finland. Authors: Pekka Reponen, Sara Hihnala, Marko Rauska and Erkki Karvonen (Meehanite Technology Oy) Date:

2 CONTENT 1. Waste water treatment in a paper mill with integrated mechanical pulping mill, General Sawdust chemical pulp PM1 and Absorbex production Production of Imprex-product PM2 production line Energy production General issues concerning waste water treatment Waste water treatment Biogas production Discharges into the sea

3 List of abbreviations: BOD Biological Oxygen Demand PGW Pressure groundwood pulping ph Acidity or alkalinity of water Q Water flow SS Suspended Solids COD Chemical Oxygen Demand MFC-paper machine finished coated paper PM1 and PM2 Paper machines 1 and 2 T Temperature TMP Thermomechanical pulping UASB Upflow Anaerobic Sludge Blanket VOC Volatile organic compound(s) WWT Waste Water Treatment MBBR Moving Bed Biofilm Reactor RCF Recycled fibre 3

4 1. Waste water treatment in a paper mill with integrated mechanical pulping mill, 1.1 General Pilot mill is a forest products company in Finland. The company specializes in laminating paper, matt coated bulky paper and sawn products. The pilot mill has two production plants in Finland and a subsidiary in Malaysia. The annual production capacity of pilot mill is tonnes of Absorbex Kraft Paper and tonnes of Imprex products, tonnes of Solaris paper, and m3 of sawn timber. The company employs 570 people (see Figure 1). The pilot mill produces chemical pulp used in paper production from saw- dust. Collected cardboard and paperboard (RCF) will replace some of the softwood sawdust. TMP plant uses also sawmill chips as rawmaterial. TMP is a furnice in bulky paper production. Peroxide/hydosulfite is used in the bleaching. Figure 1. Case pilot mill in Finland. 4

5 1.2. Sawdust chemical pulp Sawdust is screened before feeding it into the digester where oversize particles are crushed. The mill has two digesters. The heated coocing liquor is fed to the sawdust including some coocing emphasizing chemicals. After the defibrator the pulp is led to a digester and from there the pulp is fed to the blowtank. Raw turpentine is recovered by the heat recovery and concentrated odorous gases are going to the biological waste water treatment. The torch is here a spare equipment. The pulp will be washed in two washing lines. After sorting, the pulp is led to a storage tower and used in PM 1. Black liquor is evaporated to 75 % dry solid. Chemicals are regenerated by burning in the recovery boiler. The organic material will be burned. Unorganic material will be in the bottom of s boiler and diluted with white liquor. This green liquor is regenerated in a causticizing department. The created heat is used for electrical power generation. The bottom sludge from the green liquor tank is taken to the dumpsite. Lime milk from lime slaking is fed to causticizing tank and after that to Eco-filter. The filtrate is white liquor. Before digesting, the make up chemicals are added to white liquor. Lime sludge is burned in lime sludge kiln to lime. The fuel in lime kiln is natural gas and biogas from the anaerobic waste water treatment PM1 and Absorbex production The furnice in Absorbex-paper production is sawdust cellulose and the own recycled pulp RCF. Absorbex-paper production process is a normal paper production process including pulp dosage, pulp refining and additives and fillers. Forming of paperweb and wetpressing, webdrying, calandering and paper rolling. And following slitting and finishing Production of Imprex-product In Imprex-process the raw paper is impregnated by resin in impregnation pond and dryed by floating webdryer. The final product is packed water resistant or sheeted PM2 production line TMP-production. Spruce chips from sawmills are refined and bleached with hydrosulfite or peroxide. Kraftpulp will be purchased. Solaris paper produced in PM2 is MFC-paper (machine finished coated).paper production process is normal paper production process including coating process. Coating color kitchen is also included in the process. 5

6 1.6. Energy production The fuel in the combined cycle power plant is natural gas. Both electrical power and vapor are made by the system. The recovery boiler produces vapor and it can be used also for electrical power production in turbine General issues concerning waste water treatment With the combination of anaerobic and aerobic biological treatment, a significant reduction of the organic load of wastewater can be achieved. A simplified scheme of a combined anaerobic/aerobic waste water treatment plant is shown in Figure 2. Figure 2. Flow-chart of the combination of anaerobic and aerobic waste water treatment plant in the pilot mill. Following anaerobic reactor systems can be used: a contact reactor, a UASBreactor (Upflow Anaerobic Sludge Blanket) like in the pilot mill, a fixed-bed reactor and a fluidised bed reactor. UASB reactor process is shown generally in Figure 3. 6

7 Figure 3. Upflow Anaerobic Sludge Blanket reactor process. Biogas is mainly a mixture of methane and carbon dioxide and it can be used as energy source. Compared to the aerobic waste water treatment, much less biomass is produced during the anaerobic degradation process Waste water treatment The pilot mill has both the aerobic and anaerobic waste water treatment systems. Only a small amount, circa 10% of all waste water flow will be treated anaerobically. The total waste water amount is about m 3 /day. The process flow-chart is presented in Figure 4. 7

8 Figure 4. Aerobic and anaerobic waste water treatment process flow-chart in the pilot mill. Waste waters from mills with high COD-concentration are collected to the primary flotation. Suspended solids are removed with the air flotation and the clarificate is led to a tank where also concentrates from evaporation plant and other clarificates from the mill are led. The anaerobic treatment can be used because of the high COD-concentration in the waste waters. Waste water will be cooled in the two cooling towers suitable for biological treatment (about 37 C, Figure 5.). 8

9 Figure 5. Cooling towers at the pilot mill. The following process is the anaerobic acid stage where hydrolytic microorganisms degrade polymer type materials such as polysaccharides and proteins to monomers (Figure 6). The monomers are then converted into fatty acids (VFA) with a small amount of H2. The primary acids are acetic, propionic and buturic with small quantities of valeric. In the acidification stage a minimal reduction of COD occurs. In methane stage the organic acids are breaking down with methane bacteria to methane and carbondioxide in mesofilic range (T=29-38 C,pH=7). In this upflow the anaerobic sludge blanket (UASB) process wastewater is directed to the bottom of the reactor tank where it must be distributed uniformly. The wastewater flows upwards trough the blanket of biologically formed granules which consumes the waste as it passes through the blanket. Methane and carbon dioxide gas will rise and are captured in the gas dome. Liquid passes into the settling portion of the reactor tank where solid-liquid separation takes place. Excess granules can be taken to the granule tank for spare. COD reduction should be about 80 %. 9

10 Figure 6. Anaerobic reactor. Anaerobically purified waste water will be led to the aerated basin (1500 m 3 ) together with other waste waters from paper and pulp mills via primary clarifier (see Figure 7). Aerated basin is 6 m deep and the area is m 2. In the aeration basin there are Frings bottom aerators and Aqua turbo surface aerators. The aeration basin is a part of the active sludge process, where the aerobic bacterias will use organic material and they will increase. As a result sludge is formed. After the aeration basin the purified waste water will be led to the sedimentation basin. Figure 7. Aerated basin and surface and bottom aerators. 10

11 Waste water passes after the aeration basin to the secondary sedimentation where biosludge will be thickened in the gravity thickener (see Figure 8). All sludges are dryed with Tasster screw presses and belt presses into %. The purified water is led to the sea. Figure 8. The sedimentation basin Biogas production Anaerobic waste water treatment started in 1990 in pilot mill. Methane content has been an average 82 %. The gas is used for the energy production. 2. Discharges into the sea Environmental requirements to be met according to the permit. Effluent 21800m3/d permission Suspended solids 0,7t/d BOD 0,3t/d 2t/d COD 2,9t/d 7t/d Nitrogen 112,6kg/d 160kg/d Phosphorus 14kg/d 20kg/d 11