Capital costs of HSSF CWs are approximately the same as those of conventional treatment systems (activated sludge, rotating biological contactors). The cost varies widely among countries, and therefore, it is useless to mention the prices. The total capital cost usually consists of three major components: pretreatment (approx. 25%), filtration bed (approx. 60%), and remaining 15% is the cost of'other components' such as distribution and collection systems, distribution boxes, fencing, etc. The cost of
the filtration bed is composed of the cost of the filtration material and its transportation (40%), isolation (10%), earth work (5%), and planting (5%).
On the other hand, the O&M costs of HSSF CWs are much lower as compared to conventional treatment systems due to absence of any mechanical parts and in most
Fish cases the absence of electricity. The maintenance is very limited but essential especially for pretreatment units which must be regularly checked and emptied. Also, distribution on wastewater and water level in the filtration beds must be carefully checked on a regular basis.
Comparison with conventional treatment systems
Nearly 40 years of operation have revealed many advantages of HSSF CWs as compared to conventional treatment systems.
They can treat successfully diluted wastewater with low concentrations of organics (BOD5). While activated sludge-based treatment plants require some minimum concentrations of BOD5 (50-80 mgl~ ) to keep the activated sludge in healthy conditions, HSSF CWs can treat wastewater with BOD5 concentrations well below 20mgl_1 (Table 3, Figure 23).
The cope well with water quality and quantity fluctuations and HSSF CWs can operate intermittently. Therefore, they could be used for summer houses, camping sites, seasonal restaurants, and recreational objects. They require much less maintenance but regular maintenance is absolutely necessary. HSSF CWs is a robust technology that only rarely fails ifwell designed and built. Also, they fit nicely into the landscape.
On the other hand, there some disadvantages. HSSF CWs require more land than conventional treatment systems. If designed for removal of organics and suspended solids, removal of ammonia and phosphorus is very low in HSSF CWs (Tables 1 and 3). If they fail, it is difficult to find the problem and to restore the process. In conventional treatment systems the problems are usually more obvious.
HSSF CWs have become a viable alternative to conventional treatment systems for various types ofwastewater. The exact number is not known but it has been estimated that at present about 60 000 HSSF CWs are in operation around the world with the majority being in Europe.
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