13. Advanced treatment processes, Danish Environmental Protection Agency

Feminisation of fish

13. Advanced treatment processes

Conventional sewage treatment is not an effective barrier to trace contaminants with e.g. high estrogenic potency. Removal rates published in the literature vary greatly depending on e.g. local treatment facilities, discharges from the industry and the nature of the contaminant as discussed in the previous chapters (102). There has, therefore, been an increasing focus on the use of more advanced treatment processes with the objective to remove the trace contaminants and among them the estrogenic compounds.

The effectiveness of electrochemical methods in purification of synthetic waste waters containing bisphenol A has been tested in a study by Boscoletto et al. (256). The concentration of bisphenol A ranged from 20 to 2,000 mg/l and are thus considerably higher than the concentrations found in the effluent of STPs of 4.8- 4,000 ng/l. Electrochemical tests were conducted galvanostatically. A titanium mesh was used as a cathode and a platinium mesh or a titanium-supported lead dioxide film as anode. The treatment was performed in 2.8 % NaCl at pH >10.6 and the effect of different times of electrolysis was tested. Complete destruction of bisphenyl A was detected after 24 h. The final products consisted of simple short chain aliphatic acids. However, formation of chlorinated aromatic intermediates was observed during the treatment process. Boscoletto et al. (256) concluded that further investigation was necessary a.o. to find the optimal electrolysis conditions leading to a reaction path free of dangerous products.

Enzymatic treatment represents one method by which selective removal of contaminating compounds may be obtained and has been widely studied (Aitjen 1993 in (257)). Caza et al. (257) performed a study with the objective to optimize the reaction parameters, in unbuffered tap water, to achieve at least 95 % removal of several different aromatic compounds including bisphenol A by using soybean peroxidase (SBP). The reaction parameters, which were optimized, were pH, SBP dose both in the presence and absence of polyethylene glycol (PEG), hydrogen peroxidase to substrate ratio (H₂O₂/substrate) and PEG dose. The investigation demonstrated that it was possible to optimise the treatment parameters to achieve the wanted removal efficiencies. However, despite the suitability of enzymatic methods in the treatment of clear water with solutions of e.g. bisphenol A, the potential for STPs effluents is questionable.

Essex and Suffolk Water in the U.K. investigated estrogenic aspects of its treated wastewater-recycling scheme. The waste water was discharged directly into the Hanningsfield reservoir for a period of one year after UV disinfection as a temporary recycling scheme. It was the intention that a permanent recycling scheme with output from a new STP would be discharged into the River Chelmer from which water is abstracted. A research programme was carried out which included investigations of the effect of advance treatment processes on the removal of AP, APnEO, estradiol, estrone and ethinylestradiol. A pilot plant treatment of spiked sewage effluent showed that the plant removed substantial amounts of the compounds. The pilot plant included pre-ozone (1 mg/l for 4 min), ferric sulphate clarifier (8 mg/l, pH 6.2-6.5), sand filter, post-ozone followed by granular activated carbon (GAC). The pilot plant flow was 2,000 L/h. Monitoring for estrogens in the inlet and outlet of the UV disinfection plant before discharging the water indicated that UV light could reduce the estrogenicity. Laboratory experiments with estrone, estradiol, and ethinylestradiol standards showed removal in the range of 4-24 % after UV-treatment. The treatment parameters were 145 m Ws/cm² and 20 s retention time (101). The initial concentration of each steroid was approx. 25 ng/l, which is within the range of concentrations found in effluents of STPs. Furthermore, application of activated carbon (50 mg/l) resulted in a mean removal of estrogens from an operated pilot plant of 94.4 % during a period of eight days.

Shishida et al. (100) compared the capability of sand filtration, microfiltration (MF), reverse osmosis (RO) and ozone/hydrogen peroxide treatment (AOP) to reduce the estrogenicity and genotoxicity of the secondary effluent from a municipal STP with a pilot-scale reactor. A stream of the effluent water was let to the pilot plant. The water was pumped into the following sequence of treatment steps: sand filter unit, and an AOP reactor and a membrane treatment unit in parallel. The membrane treatment unit consisted of two subunits: a MF unit followed by a RO unit. Samples were collected at the outlets of secondary clarification, sand filtration, AOP, MF and RO units. The reduction of the toxicity in the waste water was evaluated by testing with different bioassays. The results of the bioassays showed that both AOP and RO treatments effectively reduced genotoxicity, cytotoxicity and estrogenicity of the secondary effluent from the full scale STP. No significant differences were observed among the secondary effluent, sand filtration and MF effluents with respect to those toxicities. Shishida et al. (100) concluded that the installation of sand filtration and MF modules in a STP is not sufficient for the reduction of those toxicities.

An Australian three-year project: "Optimised Use of Membrane Hybrid Processes for Water Recycling" (ARC SPIRT Project) was initiated in 2000 with the Queensland Government as the industry partner. Endocrine disrupter removal is the core issue of this project. The aims of the project are to investigate trace contaminant removal by hybrid membrane processes from waste waters (102).

A number of processes have been investigated regarding their potential for removal of endocrine disrupters. Those processes are ferric chloride coagulation, powdered activated carbon, magnetic ion exchange combined with microfiltration (MF) or ultrafiltration (UF) as well as nanofiltration (NF) and reverse osmosis (RO). The key findings have been a negligible removal (<10 %) of estrone with ferric chloride coagulation and very high removal (>90 %) with powdered activated carbon. Magnetic ion exchange varied from 40 to 70 % removal dependent on solution chemistry and dissociation of the hormone. Nanofiltration showed an initial retention of 70-95 % but, for most membranes, this retention dropped significantly after an initial filtration period. For some reverse osmosis membranes, retention was similar to nanofiltration, but others showed a very high and stable retention of the compounds. Microfiltration also showed initial almost complete retention followed by a drop as expected. The presence of matrix compounds from water and waste waters affected retention for some membranes. The results showed adsorption of polar contaminants to materials used in the treatments. This might be a significant risk in water recycling, in which contaminants accumulate to comparably high quantities and may be released during treatment. This requires further investigations. The obtained results are currently being confirmed on larger scale systems (102).

A three-year ongoing EU study (POSEIDON) is working with the development of possible clarification techniques for increasing the removal of endocrine disrupters. Ried & Mielcke (103) have studied the application of ozone and UV. Pilot tests were carried out at the municipal STP in Braunschweig in Germany. Braunschweig STP is designed for 385,000 PE and comprises different treatment steps with mechanical pre-treatment and biological treatment for carbon and nutrient removal. Under normal conditions, the flow rate is approx. 60,000 m³/d. Braunschweig STP is the potential end user of the EU project POSEIDON. The tested ozone/UV pilot plant installed at the STP comprises an ozone generator (100 g/h), 2 diffuser/bubble columns followed by an UV-reactor. The initial ozone treatment improves the conditions for the following UV-treatment by decreasing absorption coefficients, which eventually improves the UV transmittance from 59 % to 84 %. The preliminary results show a positive treatment effect as regards endocrine disrupters. The operating costs of the production of ozone were estimated to be between 0.90 and 1.60 EURO per kg ozone depending on the energy prices and the system capacity.