Final Treatment Behind the Secondary Clarifier

The methods of industrial and municipal wastewater treatment have already achieved a high standard by combining biological and mechanical processes and additional treatment steps for the elimination of phosphorus and nitrogen (see Chapter 10). However, the microbiological quality of effluents is still a burden for surface waters. Pathogens like bacteria, viruses and parasites are a problem, particularly for bathing waters in summer, or in areas with drinking water reservoirs. There are different standards in effect, such as the EU Water Framework Directive (EU 2000) with the aim to reach a high ecological water quality by 2015, or the EEC Council Directive 76/160/EEC of 8 December 1975 concerning the quality of bathing waters (EEC 1991; Table 12.6).

Ultra- and microfiltration are of high interest as a post-treatment method after sedimentation, and recently, as an alternative to the traditional gravity settlers used in municipal wastewater treatment to meet the quality standards mentioned above.

The high efficiency of membrane systems as a final treatment in municipal WWTP was investigated and proven by Altmann et al. (1995) in Berlin-Ruhleben. In pilot-scale experiments, five different membrane systems were investigated in combination with phosphate precipitation. Microorganisms were found to be eliminated by a factor of several orders of magnitude and viruses were typically adsorbed on suspended solids, which were eliminated (see Fig. 12.10 in Section 12.5.2). An alternative is to couple biological treatment with sand filtration.

Table 12.6 Requirements for post treatment of effluents from WWTP Microbiological Standard in EEC Council Directive 76/160/EEC of 8 December 1975 concerning the quality of bathing water (EEC 1991).

Parameter Imperative value Guide value

Total coliform bacteria 500 10000

(cells/100 mL)

Fecal coliform bacteria 100 2000

(cells/100 mL)

Streptococcusfaecalis 100

(cells/100 mL)

Salmonella - 0

Bowel viruses - 0

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