scharged water. To meet these standards, a combination of anaerobic and aerobic is required, often coupled to nutrient removal systems.most of the air pollution is related to fossil energy consumption, prevention as a method to reduce environmental pollution is even more important than it is for wastewater . For some components (eg VOC, dust) methods exist for the treatment of polluted air, however frequently at high costs [32].
10. Economic considerations
The costs of wastewater treatment are a factor of major importance for the selection the appropriate treatment system. Estimates should be made of the investment costs and the expected annual costs. The investment costs are largely determined by construction costs, the costs of land and the required degree of removal of pollutants. The annual costs will depend on the price of the energy and chemicals required for the operation of the plant, the discharge fees and the capital costs on investment. A problem for the estimation of the costs of treatment plants is that prices are rapidly changing. Cost estimates should therefore be referenced to an index [34]. A comparison of the costs of 6 treatment systems (stabilisation ponds; aerated ponds; high rate anaerobic treatment + ponds; high rate anaerobic treatment + trickling filters; activated sludge process; and oxidation ditch; DHV, 1993) it can be concluded that high rate anaerobic treatment + post treatment of the effluent offers a very economic and effective solution [32]. relatively high initial costs are compensated for by the low costs of energy and maintenance, which results in low running costs and a limited need for land. Costs of a stabilisation pond, high-rate anaerobic treatment plant + post-treatment in a pond and an activated sludge process for the sewage treatment plant for a town of 50,000 inhabitants (producing ca 550 ton BOD and ca 135 ton N) given as a reference, are: resp. around 3,5.10 6 , 2.10 6 and 2.10 6 USD for investment costs and running cost resp. around 400,000, 300,000 and 430,000 USD on an annual basis. In this calculation it is assumed that electricity costs are 0.10 USD per kWh, sludge disposal costs 10 USD per 1000 kg and that the price of land is 25 USD per m 2 . Lagoons will become more economical if land costs are below 10 to 20 USD/m 2 . Wastewater from slaughterhouses, tanneries, and the dairy industry are more heavily loaded with pollutants than sewage. This will have the effect that anaerobic processes are more competitive than aerobic processes owing to the much lower energy costs of ...
