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The detailed composition of wastewater has significant impact on the biological wastewater treatment processes. The carbon sources present govern the oxygen uptake rate, the rate of denitrification and the biological phosphorus uptake rate. In this paper, the respiration rate determinations used for the bi- or tri-substrate determinations are coupled to removal capacity in order to develop rate-capacity diagrams, that gives a fuller picture of the wastewater and its influence on the biological processes. The directly degradable carbon source gives high reaction rates, but the concentration and thus the capacity can be very limited. In such a case the overall obtainable removal rate is of more interest than the peak rate. The rate-capacity diagrams are shown for raw, primary settled and primary precipitated wastewater, characterized by detailed investigations of the respiration rates obtainable. It shows the effect of pretreatment not only on the total concentrations in the pre-treated wastewater, but also in the rate-capacity curves for biological denitrification. The rate-capacity curves are in practice coupled to the design and operation of the treatment plant. These two factors determine whether the carbon sources in the influent wastewater are used for denitrification (or biological phosphorus removal), for oxidation with oxygen or for sludge production.

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Biological sludge hydrolysis was demonstrated in full scale at three Danish wastewater treatment plants. For primary sludge the hydrolysis yield expressed in terms of filtrable COD varied from 9–16% of the total COD in the sludge (WTPs 1 & 2) and for the hydrolysis of activated sludge a yield of 2.5% of the total COD was found. The addition of hydrolysate was demonstrated to improve the biological P removal considerably. No effect on the nitrogen removal could be identified due to a favourable wastewater composition during the demonstration phase. A cost-benefit analysis showed that biological sludge hydrolysis may be a cost efficient process that should be considered in connection with the upgrading of wastewater treatment plants to nutrient removal.

In recent years the grey-box modelling approach has been applied to wastewater transportation and treatment. Grey-box models are characterized by the combination of deterministic and stochastic terms to form a model where all the parameters are statistically identifiable from the on-line measurements. With respect to the development of software sensors, the grey-box models possess two important features. Firstly, the on-line measurements can be filtered according to the grey-box model in order to remove noise deriving from the measuring equipment and controlling devices. Secondly, the grey-box models may contain terms which can be estimated on-line by use of the models and measurements. In this paper, it is demonstrated that many storage basins in sewer systems can be used as an on-line flow measurement, provided that the basin is monitored on-line with a level transmitter and that a grey-box model for the specific dynamics is identified. Similarly, an on-line software sensor for detecting the occurrence of backwater phenomena can be developed by comparing the dynamics of a flow measurement with a nearby level measurement. For treatment plants it is found that grey-box models applied to on-line ammonia measurements from the aeration tank of an alternating plant provide information on the incoming ammonia load. It is also shown how measurements of the return sludge concentration from a secondary clarifier can be filtered to minimize the effect of the scraper. Thus, important information can be derived from on-line measurements if the appropriate grey-box model for the specific system is identified.

A Discfilter with 10 and 18 microm filter openings, respectively, was placed in parallel to a flotation plant for separation of biological flocs from a post-denitrifying Kaldnes Moving Bed Process, the last treatment step at the municipal wastewater treatment plant at Sjöunda, Malmö, Sweden. The effluent concentrations from the 10 and 18 microm filter were 2-5 and 2-8 mg SS L(-1), respectively, which is comparable to, or better than, the flotation plant. Comparison with experiences from activated sludge plants shows that the Discfilter works especially well after the Kaldnes process. Particle size distribution (PSD) studies show that particles larger than the filter openings of 10 and 18 microm are separated with approximately 90% efficiency, whereas most of the smaller particles pass the filter. This fact indicates that the major particle separation mechanism is physical blocking. These findings point to the possibility of improving the prediction of the separation efficiency by combining measurements of turbidity and suspended solids with particle size analysis.