Economic Evaluation of Small Capacity Sewage Processing Units

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 are: 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, operation and construction costs are calculated as a unit capacity function
for a 40-year operating period. Cost calculation is based on the analysis of its several
components such as energy consumption, chemicals 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.
Cost estimation is based, mainly, on the calculation of the several cost elements’ quantities,
with the development of standardized plans. Total and partial costs are expressed as
functions of plant capacity. The annual operation and maintenance (O&M) cost, the project
cost, as well as the total construction, maintenance and operation cost for 40 years of
operation (Present Value) can be written as a function of flow rate, in the form of a+bQ+cQ2;
whereas energy cost can be described by a linear function of flow rate, in the form of 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 positive scale economies for the studied region of cost per
E.P.; this trend was especially evident for plant capacities up to 5,000 E.P.
Natural wastewater treatment methods present the least expensive option for the examined
range of E.P. 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.
The results provide a first level of information, which could be utilized for a quick financial
evaluation of the aforementioned systems as well as in the general decision making plan.