Capital and OM costs

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%),

Figure 21 HSSF CW for treatment of wastewaters from production of aniline and nitric acid in Estarreja, Portugal. Photo by J. Vymazal.

Figure 21 HSSF CW for treatment of wastewaters from production of aniline and nitric acid in Estarreja, Portugal. Photo by J. Vymazal.

Figure 22 HSSF for treatment of landfill leachate near Wilmington, NC, USA. Photo by J. Vymazal.

filtration bed (approx. 60%), and remaining 15% is the cost of 'other components' such as distribution and col lection systems, distribution boxes, fencing, etc. The cost of the filtration bed is composed of the cost of the filtra tion 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 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 advan tages 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 J) to keep the acti vated 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 fluc tuations and HSSF CWs can operate intermittently. Therefore, they could be used for summer houses, camp ing sites, seasonal restaurants, and recreational objects. They require much less maintenance but regular main tenance is absolutely necessary. HSSF CWs is a robust technology that only rarely fails if well 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 sys tems. 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 con ventional treatment systems for various types of wastewater. 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.

Figure 23 HSSF CW at Krovi, Czech Republic, treating wastewaters from combined sewer sytem. Photo by J. Vymazal.
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