Author(s)

M. Gratziou & M. Tsalkatidou

Abstract

This paper aims at a comparative evaluation of the total cost of urban sewage processing units for several treatment systems, appropriate mainly for small capacity plants. The studied systems were: Oxidation Ditch (O.D.), Trickling Filter (T.F.), Rotating Biological Contactor (R.B.C.), Compact Sequential Batch Reactor (S.B.R.), Waste Stabilization Ponds (S.P.) and Subsurface Constructed Wetland Systems (C.W). For each system, operating and construction costs are calculated as flow rate functions for a 40-year operating period. Cost calculation is based on the analysis of its several components such as energy consumption, chemical consumption, personnel salaries, maintenance expenses, construction materials and their respective quantities, required mechanical equipment and land value. All pecuniary flows, in order to be comparable, are converted into current value. For each case total and partial costs are expressed as functions of the plant capacity. The annual operating and maintenance (O&M) cost, the project cost, as well as the total construction, maintenance and operating cost for 40 years of operation (Present Value) can be expressed as functions of flow rate, of the form a+bQ-cQ 2 ; whereas energy cost can be described by a linear function of flow rate, of the form a+bQ, with excellent approximation. The coefficients a, b, c, for the various treatment methods examined and for every cost category are summarized in tables. All treatment methods exhibited, for the several categories of cost per E.P. examined, strong positive scale economies, especially for plant capacities up to 5000 E.P. Wastewater treatment by natural methods is the least expensive choice in every case. The classification of the treatment methods from the least to the most expensive one depends on plant capacity. The classification of the treatment methods based on their cost for different plant capacities is provided. Keywords: cost evaluation, domestic wastewater, processing units.

Keywords

cost evaluation, domestic wastewater, processing units.