Optimization of a large industrial wastewater treatment plant using a modeling approach: A case study.

The objective of this paper is to find the optimum solid retention time (SRT) of a wastewater treatment plant (WWTP), which minimizes operating costs, using a modeling approach with WEST software by MIKE DHI®. For the determination of the kinetic and stoichiometric parameters (used for the correct calibration of the model implemented), respirometric and kinetic batch tests were carried out. Each Oxidation ditch was modeled by a sequence of four aerated activated sludge units (ASUs) and four anoxic ASUs with recirculation. The model is able to simulate the separation efficiency of the secondary settler, which is generally quite low: in fact, the industrial origin of the wastewater induces the formation of small flocs, the dimensions of which can be further reduced by the presence of surface aerators. The nitrification/denitrification process is also accurately predicted. Using data obtained from the model, mass balances at the steady state for COD and N were made and compared to the ones obtained using measured data. After calibration and validation of the model, steady-state simulations were carried out by increasing and decreasing the SRT of the system under two different operational conditions used by the managing company and by evaluating the costs related to the water treatment line and the sludge treatment line for each scenario. It is interesting to note how the total costs are lower in summer than in winter (7.2 €cent/m3 in summer and 8.7 €cent/m3 in winter, in scenario 0). In general, the increase in the SRT led to a decrease in the total management costs. In fact, differences between scenario 0 and the scenario with the lowest total treatment costs (corresponding to an SRT of 11.4 d in winter and 10.0 d in summer) could give rise to total savings of about 44·000€/year in summer and 93·000€/year in winter.

Environmental comparison of alternative treatments for sewage sludge: An Italian case study.

A Life Cycle Assessment (LCA) was applied to compare different alternatives for sewage sludge treatment: such as land spreading, composting, incineration, landfill and wet oxidation. The LCA system boundaries include mechanical dewatering, the alternative treatment, transport, and final disposal/recovery of residues. Cases of recovered materials produced as outputs from the systems, were resolved by expanding the system boundaries to include avoided primary productions. The impact assessment was calculated using the CML-IA baseline method. Results showed that the incineration of sewage sludge with electricity production and solid residues recovery collects the lowest impact indicator values in the categories human toxicity, fresh water aquatic ecotoxicity, acidification and eutrophication, while it has the highest values for the categories global warming and ozone layer depletion. Land spreading has the lowest values for the categories abiotic depletion, fossil fuel depletion, global warming, ozone layer depletion and photochemical oxidation, while it collects the highest values for terrestrial ecotoxicity and eutrophication. Wet oxidation has just one of the best indicators (terrestrial ecotoxicity) and three of the worst ones (abiotic depletion, human toxicity and fresh water aquatic ecotoxicity). Composting process shows intermediate results. Landfill has the worst performances in global warming, photochemical oxidation and acidification. Results indicate that if the aim is to reduce the effect of the common practice of sludge land spreading on human and ecosystem toxicity, on acidification and on eutrophication, incineration with energy recovery would clearly improve the environmental performance of those indicators, but an increase in resource depletion and global warming is unavoidable. However, these conclusions are strictly linked to the effective recovery of solid residues from incineration, as the results are shown to be very sensitive with respect to this assumption. Similarly, the quality of the wet oxidation process residues plays an important role in defining the impact of this treatment.