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Sunday, January 18, 2009

The treatment of hospital wastewater: an appraisal

Health Stream Literature Summary - Issue 46 - June 2007

The treatment of hospital wastewater: an appraisal

Pauwels, B. and Verstraete, W. (2006) Journal of Water & Health, 4 (4) 405-416.

 

Hospitals discharge large amounts of chemicals and microbial agents into wastewater. Chemicals discharges include pharmaceuticals, iodinated X-ray contrast media (ICM: used for X-ray imaging of soft tissues), antibiotics and disinfectants. Hospitals also discharge antibiotic resistant bacteria, viruses and maybe even prions etc. While some of these contaminants are unique to hospital settings, many are also present in sewage from domestic residences. Many of the chemical compounds are resistant to wastewater treatment and may end up in surface water where they may influence the aquatic ecosystem and interfere with the food chain. This may also pose a risk of human exposure through drinking surface water. This paper examines the issue of hospital wastewater and whether there is a case to justify separation of some or all hospital waste from the normal sewage stream.

 

The most important chemicals in hospital wastewater are antibiotics, cytostatic agents, anaesthetics, disinfectant, platinum, mercury, rare earth elements (gadolinium, indium, osmium) and iodinated X-ray contrast media. Other pharmaceuticals which have been found in wastewater treatment plant (WWTP) effluents include lipid regulators, analgesics, antibiotics, antidepressants, antiepileptics, antineoplastics, antipyretics, antiphlogistics, antirheumatics, ß-blockers, broncholytics, ß2-sympathomimetics, estrogens, secretolytics, vasodilators and X-ray contrast media.

 

Of the antibiotics used for human purposes in Europe , 26% are used in hospitals. These antibiotics and their metabolites are excreted with urine and faeces and end up in WWTP. Efforts are being made to evaluate the risk of antibiotics however there is a lack of concentration level data in hospital wastewater. For each medical treatment about 100g of X-ray contrast media is used. This represents about 30g of absorbable organic iodinated compounds (AOI). The AOI are biologically inert and stable toward metabolism during their passage through the body. They are excreted almost completely within a day after administration and end up in the WWTP where they are not well removed. As not much is know about their fate and long term effects, there is a risk associated to their spread in the environment. Particularly of interest are oestrogens. A UK study observed the feminisation of male caged fish at discharge sites of WWTPs. On a daily basis, women excrete on average approximately 32, 14 and 106 microg of conjugate estrone (E1), estradiol (E2) and estriol (E3), respectively. Pregnant women excrete about 100 times this amount. Hospital wastewater probably has elevated concentrations of E1, E2 and E3, however hospital wastewater oestrogen levels have not been reported.

 

Hospital wastewaters are a source of bacteria with acquired resistance against antibiotics with a least a factor of 2-10 higher than domestic wastewater. Bacteria become resistant to a specific antibiotic by transfer of genes encoding for this resistance being transferred vertically to the bacteria's offspring or horizontally, among bacteria of different taxonomic affiliation. Gene transfer is optimal at high cell densities and under high antibiotic concentrations. However under heterogenous environmental conditions, this gene transfer can still occur at significant levels. The emergence and spread of methicillin-resistant Staphylococcus aureus (MRSA) is of particular concern.

 

Removal efficiencies of different wastewater treatment techniques for various hospital related pollutants were reviewed from the literature. There was 50-99% removal of antibiotics by powdered activated carbon/granular activated carbon (PAC/GAC), greater than 95% by ozonation, 50-80% using ultraviolet photolysis and greater than 90% using reverse osmosis. Antibiotic resistant propagules (viable bacteria) were reported to be removed by less than 1, 2 and greater than 3 log units by activated sludge in various studies. Therapeutic drugs had a 90-99% removal by PAC/GAC, poor to greater than 95% removal using ozonation and 50% to greater than 90% removal with reverse osmosis. Iodinated contrast media removal by activated sludge varied from none to 85% and by ozonation from poor (14%) to greater than 80%. Oestrogens were removed greater than 99.8% with PAC/GAC and 95-99% using reverse osmosis.

 

There are four possible scenarios for hospital wastewater treatment and disposal: (1) direct discharge to the environment, (2) co-treatment in a municipal WWTP, (3) on-site wastewater treatment and subsequent discharge of the effluent to the environment and (4) first on-site and subsequently municipal wastewater treatment. A risk assessment for the hazard posed by the hospital wastewater needs to be conducted for all 4 scenarios. Scenario 3 could possibly provide the highest efficiency and environmental benefits.

 

Membrane bioreactors (MBRs) have been proposed as a potential alternative for conventional activated sludge treatment. MBR may play a key role in hospital wastewater treatment because of the high removal of bacteria. There are several post-treatment technologies such as activated carbon, ozonation and UV photolysis which remove hospital related pollutants quite well. Reverse osmosis is practically not possible and advisable because of the required pre-treatment of WWTP effluent before using this technique and because of the generation of concentrated sidestreams. Ozonation is relatively cheap but by products are poorly characterised. A proposal had been suggested for source separation of urine of patients which have undergone X-ray imaging. The urine containing ICMs could be processed as chemical waste. This source separation could also be applied to the urine of pregnant women in hospital maternity departments. This urine could be treated in a small scale WWTP which has been enriched with oestrogen degrading organisms. The economic and social feasibility of this needs to be demonstrated.

 

There is a lack of data about the possible impacts of hospital discharges, direct or indirect on the environment. There is a need to develop treatment scenarios for hospital wastewater with regard to attainable efficiency and costs per m 3 of water treated. The idea of uncoupling hospitals from public sewers requires thorough investigation by technologists, ecotoxicologists and public health specialists.

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