Advanced disinfection of wastewater ponds effluent by UV irradiation. T. Fuhrmann and K.-U. Rudolph

Transcription

1 Advanced disinfection of wastewater ponds effluent by UV irradiation T. Fuhrmann and K.-U. Rudolph 1

2 Pathogen reduction in wastewater ponds Pond systems offer excellent treatment features in terms of physical and biological reduction of pathogens Typical pathogen reductions (bacterial indicators): about 3 log units and more Escherichia coli concentrations in influent and effluent of different ponds in Germany 3 log units 2

3 Subsequent disinfection of pond effluent Despite the significant disinfection effects of pond systems, pond effluents are variable site-specific (about 1.5 to 4.5 log units) seasonal fluctuations With ponds as sole treatment step, effluents regularly do not fulfil relevant micro-biological quality requirements, e. g. for wastewater reuse as other conventional treatment systems, too! Constructional improvements (buffers etc.) Need for supplementary effluent disinfection = subsequent disinfection systems 3

4 Subsequent disinfection of pond effluent Comparison of subsequent disinfection methods (partly from Bixio and Wintgens, 2006, and Rudolph, 2006) Taking all aspects into consideration, UV irradiation has been identified as most appropriate disinfection method for pond effluents relatively simple in O&M with moderate investment and operational expenditures 4

5 Disinfection by UV irradiation Germicidal effect of UV light on micro-organisms UV light with a wavelength of 254 nm disinfects especially effectively Performance of the germicidal effect of UV radiation generally depends on the UV dose (UV dose = irradiation intensity x exposure time) 5

6 Disinfection of partly purified wastewater UV light interacts with the materials it encounters through absorption, reflection, refraction and scattering UV dose is mainly influenced by the quality of the irradiated water Optimal UV disinfection efficiency requires effluent with high UV transmittance values indicated by low SS concentration and turbidity and little colour often filtration (e.g. by sand filters) is used prior to UV disinfection Recommendations for optimal UV-dose efficacy: - suspended solids contents of 5 mg SS/l - turbidity levels of 5 NTU - UV transmittance values above 60 % 6

7 Lab scale tests on UVT UV transmittance (UVT) is the decisive parameter for design of UV irradiation units UVT mainly depends on physical processes like sedimentation and filtration applied to the water Example: Improvement of UV transmission during settlement time UV transmittance turbidity 7

8 UVT at ponds Wastewater treatment by ponds has a significant effect on the UV transmittance values Secondarily treated WWTP effluents For comparision: - secondarily treated WWTP effluents: 45 to 70 % - after specific filtration: 80 % High insolation algae loads in effluent 8

9 Tests with UV irradiation Wastewater treatment in ponds cannot ensure UVT values for optimal disinfection efficiency Laboratory scale tests with UV disinfection of pond-treated wastewater Generalised average dose-response curves from UV disinfection tests for E. coli 9

10 Tests with UV irradiation Laboratory scale tests with UV disinfection of pond-treated wastewater have shown: - reduction rates with pond effluents can be found between mechanically and advanced biologically treated wastewater - even for lower effluent qualities (UVT << 70 %) pathogen reduction adequate for agricultural reuse is feasible (e. g. E. coli values of cfu / 100 ml) - with cost-effective UV doses Depending on water quality, total costs of 0,03 0,10 EUR/m³ may be achieved. 10

11 Tests with UV irradiation UV dose of 500 J/m² (= 50 mj/cm²): typically applied with filtrated secondary effluent to achieve a target of 10 cfu / 100 ml for unrestricted irrigation average reduction to about 100 cfu / 100 ml may be achieved with pond effluent this means: the reduction is 1 log unit lower than with fully purified effluent UV dose of < 1500 J/m² (= 150 mj/cm²): ensures a reduction to 10 cfu / 100 ml the dose has to be about 3 times higher than with totally purified water still a reliable value regarding energy consumption of the UV lamps 11

12 Limitations to UV application: 1) Algae load Problems with high loads of algae SS in effluent esp. in regions with high insolation Need for algae control, e. g. by rock filters So far no clear design criteria for application of UV disinfection to algae-rich pond effluents Further research is needed 12

13 Limitations to UV application: 2) Removal of helminth eggs UV irradiation does not ensure adequate inactivation of helminth eggs Ascaris eggs are probably the most UV-resistant water-related pathogen identified to date inactivation may be less than 1 log unit with UV doses of up to 1000 J/m² (= 100 mj/cm²) (Brownell and Nelson, 2006) Applied as subsequent disinfection following a pre-treatment through ponds, this effect plays only a minor role pond system provide high removal efficiencies of helminth eggs up to 100 % By this, ponds are a most advantageous combination for UV disinfection, closing UV efficiency gap regarding helminth eggs removal 13

14 Limitations to UV application: 3) Bacterial regrowth UV irradiation produces no deposit effect like with chlorine disinfection (no residual disinfection dose) Photo / dark reactivation bacterial regrowth after UV irradiation Maximum: about 1-2 days after irradiation UV irradiation is preferably appropriate for pointof-use systems 1 2 days 14

15 Conclusions Ponds have the ability to significantly reduce pathogens, but effluents are variable and regularly does not guarantee relevant microbiological quality standards e. g. for water reuse Demand for subsequent disinfection systems Investigations with different pond effluents show UV disinfection to be a technically and economically reliable disinfection mean, even without extended pre-treatment Limitations has to be taken into consideration Further investigations necessary about UVT diminishing influences like high algae loads 15

16 Thanks a lot for your attention. and German BMBF as well as WEDECO for their support. 16