Upgrading of an activated sludge wastewater treatment plant by adding a moving bed biofilm reactor as pre-treatment and ozonation followed by biofiltration for enhanced COD reduction: design and operation experience

摘要

A paper mill producing 500,000 ton of graphic paper annually has an onsite wastewaterntreatment plant that treats 7,240,000m3 of wastewater per year, mechanically first, thennbiologically and at last by ozonation. Increased paper production capacity led to highernCOD load in the mill effluent while production of higher proportions of brighter productsngave worse biodegradability. Therefore the biological capacity of the WWTP needed to benincreased and extra measures were necessary to enhance the efficiency of COD reduction.nThe full scale implementation of one MBBR with a volume of 1,230 m3 was accomplishednin 2000 followed by another MBBR of 2,475 m3 in 2002. An ozonation step with a capacitynof 75 kg O3/h was added in 2004 to meet higher COD reduction demands during thenproduction of brighter products and thus keeping the given outflow limits. Adding a movingnbed biofilm reactor prior to the existing activated sludge step gives: (i) cost advantagesnwhen increasing biological capacity as higher COD volume loads of MBBRs allow smallernreactors than usual for activated sludge plants; (ii) a relief of strain from the activatednsludge step by biological degradation in the MBBR; (iii) equalizing of peaks in the CODnload and toxic effects before affecting the activated sludge step; (iv) a stable volumensludge index below 100 ml/g in combination with an optimization of the activatednsludge step allows good sludge separation—an important condition for further treatmentnwith ozone. Ozonation and subsequent bio-filtration pre-treated waste water provide:n(i) reduction of hard COD unobtainable by conventional treatment; (ii) controllable CODnreduction in a very wide range and therefore elimination of COD-peaks; (iii) reduction ofntreatment costs by combination of ozonation and subsequent bio-filtration; (iv) decreasenof the color in the ozonated wastewater. The MBBR step proved very simple to operatenas part of the biological treatment. Excellent control of the COD-removal rate in thenozone step allowed for economical usage and therefore acceptable operation costs innrelation to the paper production.