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

This paper reviews nutrient issues within the pulp and paper industry summarising: nitrogen and phosphorus cycles within treatment systems; sources of nutrients within pulping and papermaking processes; minimising nutrient discharge; new approaches to nutrient minimisation; and the impact of nutrients in the environment. Pulp and paper industry wastewaters generally contain insufficient nitrogen and phosphorus to satisfy bacterial growth requirements. Nutrient limitation has been linked to operational problems such as sludge bulking and poor solids separation. Nutrients have been added in conventional wastewater treatment processes to ensure optimum treatment performance. Minimising the discharge of total nitrogen and phosphorus from a nutrient limited wastewater requires both optimised nutrient supplementation and effective removal of suspended solids from the treated wastewater. In an efficiently operated wastewater treatment system, the majority of the discharged nutrients are contained within the biomass. Effective solids separation then becomes the controlling step, and optimisation of secondary clarification is crucial. Conventional practice is being challenged by the regulatory requirement to reduce nitrogen and phosphorus discharge. Two recent developments in pulp and paper wastewater treatment technologies can produce discharges low in nitrogen and phosphorus whilst operating under conventionally nutrient limited conditions: i) the nutrient limited BAS process (Biofilm-Activated Sludge) which combines biofilm and activated sludge technologies under nutrient limited conditions and ii) an activated sludge process based on the use of nitrogen-fixing bacteria. Aerated stabilisation basins often operate without nutrient addition, relying on settled biomass in the benthal zone feeding back soluble nutrients, or the fixation of atmospheric nitrogen. Thus effective nutrient minimisation strategies require a more detailed understanding of nutrient cycling and utilisation. Where it is not possible to meet discharge constraints with biological treatment alone, a tertiary treatment step may be required. In setting nutrient control guidelines, consideration should be given to the nutrient limitations of the receiving environment, including other cumulative nutrient impacts on that environment. Whether an ecosystem is N or P limited should be integrated with wastewater treatment considerations in the further design and development of treatment technology and regulatory guidelines. End-of-pipe legislation alone cannot predict environmental effects related to nutrients and must be supplemented by an effects-based approach.

Thermophilic aerobic treatment of settled pulp and paper mill effluent was studied under mill premises with two comparative pilot processes; suspended carrier biofilm process (SCBP) and activated sludge process (ASP). Full-scale mesophilic activated sludge process was a reference treatment. During the runs (61 days) hydraulic retention times (HRTs) were kept 13+/-5 h and 16+/-6 h for SCBP and ASP, respectively. Corresponding volumetric loadings rates (VLR) were 2.7+/-0.9 and 2.2+/-1.0 kg CODfilt m(-3)d(-1). Temperatures varied between 46 to 60 degrees C in both processes. Mesophilic ASP was operated with HRT of 36 h, corresponding VLR of 0.7 kg CODfilt m(-3)d(-1). Both SCBP and ASP achieved CODfilt (GF/A filtered) removals up to 85%, while the mesophilic ASP removal was 89+/-2%. NTU values were markedly higher (100-300) in thermophilic effluents than in mesophilic effluent (30). Effluent turbidity was highly dependent on temperature; in batch experiment mesophilic effluent sample had NTU values of 30 and 60 at 35 degrees C and 55 degrees C, respectively. As a conclusion, both thermophilic treatments gave high CODfilt removals, which were close to mesophilic process removal and were achieved with less than half of HRT.

A combination of the suspended carrier biofilm process and the activated sludge process (biofilm-activated sludge--BAS) has been shown to be very successful for the treatment of different types of pulp and paper mill effluents. The robust biofilm pre-treatment in combination with activated sludge results in a stable, compact and highly efficient process. Recent findings have shown that nutrient limited operation of the biofilm process greatly improves the sludge characteristics in the following activated sludge stage, while minimising sludge production and effluent discharge of nutrients. The nutrient limited BAS process was implemented at full scale at the Södra Cell Värö kraft mill and taken into operation in July 2002. After start-up and optimisation over about 5 months, the process meets all effluent discharge limits. The removal of COD is close to 70% and the removal of EDTA greater than 90%. Typical effluent concentrations of suspended solids and nutrients during stable operations have been 20-30 mg/L TSS, 0.3-0.5 mg/L phosphorus and 3-5 mg/L nitrogen. The sludge production was 0.09 kgSS/kg COD removed and the sludge volume index was 50-100 mL/g.

A process of advanced oxidation treatment discussed here has been developed in the last seven years. A technical plant for treatment of paper mill wastewater started operation in 1999. Advanced oxidation treatment is defined here as a combination of chemical and biochemical oxidation applied to a completely biodegraded effluent. The process is a combination of ozonation with following biodegradation in a biofilm reactor. The combination makes use of the effect of partial oxidation in which with reduced use of expensive chemical oxidants persistent COD becomes biodegradable. A far-reaching elimination of AOX, colour and other disturbing substances like complexing agents is achieved simultaneously. The partially oxidized compounds, now biodegradable, are eliminated in a following biofilm reactor. Using single-stage systems, COD elimination rates of up to 60% are achieved with 0.4 to 1.0 g ozone per g COD eliminated in the combined process. With a two-stage system COD elimination of 85% with 0.65 g/g has been achieved in pilot tests. The quality of the treated effluent suits fresh water requirements for most types of paper production, so that either a total closure of the water circuits or - due to increasing concentrations of electrolytes, which are not eliminated by this process - a substantial closure to about 20% of the original amount of fresh water can be achieved.

In this paper we describe an alternative flow-chart for full treatment of wastewaters rich in organic substrates, ammonia (or organic nitrogen), and sulfate, such as those generated in fish cannery industries. Biogas generated during anaerobic pretreatment of these wastewaters is rich in hydrogen sulfide that needs to be removed to enable application of the biogas. Nitrogen elimination is traditionally achieved by subsequent nitrification and denitrification of the effluent of the anaerobic reactor. Alternatively, the hydrogen sulfide in the biogas can be applied as an electron donor in an autotrophic post-denitrification step. In order to study whether sufficient hydrogen sulfide containing biogas for denitrification was produced in the anaerobic reactor, the biogas composition as a function of the anaerobic reactor-pH was estimated based on a typical wastewater composition and chemical equilibrium equations. It is demonstrated that typical sulfate and nitrogen concentrations in fish cannery wastewater are highly appropriate for application of autotrophic post-denitrification. A literature review furthermore suggested that the kinetic parameters for autotrophic denitrification by Thiobacillus denitrificans represent no bottleneck for its application. Initial experimental studies in fixed-film reactors were conducted with sodium sulfide and nitrate as an electron donor-acceptor couple. The results revealed that only moderate volumetric treatment capacities (1 g-NO3- N l(-1) day(-1)) could be achieved. Mass balances suggested that incomplete sulfide oxidation to elemental sulfur occurred, limiting biomass retention and the treatment capacity of the reactor. Future research should clarify the questions concerning product formation from sulfide oxidation.