Treatment Technologies For Middle-stage Effluent From Bamboo Sulfite Pulping

In past decades,Paper industry in China has been developed quickly,with an annual production of paper and paperboard grade about 52 million tons in the year of 2007. With the rapid growth of paper and paperboard production,the sustainable supply of fibers has become the key issue.Currently,the percentage of recycled fibers increases rapidly among fibers for papermaking in China.The chemical pulps have been become more and more important due to more application of recycled fibers and mechanical pulps.The initial resource of chemical pulps in China comes from domestic fiber production lines and imported commercial pulps from international market.For the chemical pulping lines in China,both alkali and acidic pulping technologies are applied.Nowadays,mills have paid more attention on application of sulfite pulping technologies since its byproducts,ligno-sulfate have been used widely,which is a new profit point of chemical pulping mill.China is one of countries with a lower coverage of forest,in a long terms,such situation will not be changed significantly.Therefore,the papermaking industry will focus on both wood fibers and non-wood fibers.The complete pulping process using woodchips have become more matural including the chemical recovery technologies.With containing sulfur elements in the effluent from sulfite pulping,their middle stage effluents are difficult for efficient treatment of bio-process.For treatment of effluent from bamboo sulfite pulping,there are a few investigations on this topic,and no successful process has been operated in the industry.The effluent sample are collected from bamboo pulping lines using sulfite pulping technologies in a mill,which produces commercial bamboo sulfite pulps used for producing linerboard and corrugated medium products,due to this type pulps with good bulk and high stiff properties.Cooking conditions used for bamboo pulping can be described as:chemical dosage for sodium sulfite 250 kg/t pulp,sodium carbonate 50 kg/t pulp,chips-water ratio is 1:4 and time at temperature 180 min.The extraction rate of red liquor reaches about 85%.The rest part of contaiminants will be brought into middle stage effluent by washing.There is around 60 m³ middle stage effluent discharged by producing each ton of pulp.The effluent sample was characterized as:pH 7.56,COD 2560 mg/L,BOD₅ 719 mg/L,TS 1030 mg/L,SS 476 mg/L,color 2500～3000 times,NH₃-N 498 mg/L,SO₄^2- 590 mg/L,SO₃^2- 128 mg/L,lignin sulfonate content 1189 mg/L,reducing sugar 124 mg/L.In this study,a complete treatment process has been adopted,which combines several operation units as primary clarifier-prehydrolysis-the first anaerobic treatment-the second anaerobic treatment-aerobic treatment-coagulation. Comparing the characteristics of influent and effluent to this treatment process,the quality of discharged water can meet the requirement of national regulation for effluent discharging,with COD reducing from 2560 mg/L to 348 mg/L,equivalent to a removal ratio of 86.4%;SS reducing from 476 mg/L to 140 mg/L,average removal ratio of 70.6%.