Helminth eggs removal in water reclamation: disc filtration as an effective barrier

Abstract

Increasing reuse of reclaimed water for unrestricted agricultural and landscape irrigation is raising considerable concern for human health protection due to the potential spread and contact with human parasites and pathogens. Parasites pose a significant public health concern and the removal of helminth eggs is becoming a crucial treatment step when providing safe reclaimed water. Although laboratory tests conducted so far indicate that microscreens can efficiently remove helminth eggs, convincing evidence was missing on the ability of full-scale processes to achieve such objective. This paper presents the results of a demonstration project conducted to validate the efficiency and the reliability of a gravity microscreening filtration process using the Hydrotech Discfilter, with a 10-μm pore size cloth, as a physical barrier for removing parasitic helminth eggs in full scale water reclamation plants. Tests were conducted at the Baix Llobregat Water Reclamation Plant in Barcelona (Spain) in May-June 2007. About 2 million helminth eggs of the Trichuris suis species were added to reclaimed water (7.6 m3 close recirculation system) and subsequently filtered through a Hydrotech Discfilter with 10-μm pore size cloth. A total of 68 samples of reclaimed water (34 influent samples of 20L, and 34 effluent samples of 100L) were analyzed using an adaptation of the Bailenger modified method, as recommended by WHO in “Wastewater analysis for agricultural use” by Ayres & Mara. Several changes to the modified method were applied at the Faculty of Pharmacy of the University of Barcelona, to improve the method recovery efficiency. Those adaptations resulted in an absolute recovery efficiency of up to 80-90%, considerably larger than the 30-74% recovery achieved by the recommended Bailenger modified method. Those recovery improvements were achieved by using a sample volume higher than that recommended, by omitting the grease extraction step, by analyzing total water sample volumes, and by counting all the helminth eggs present in a McMaster chamber. Results clearly showed that the Hydrotech Discfilter effluent had no helminth eggs, regardless of the helminth egg concentrations entering the filter (10–3200 eggs/20L), with the exception of one sample in which a 1egg/100L concentration was detected. Those results also confirm the high efficiency (up to 4.2 ulog for 100L samples) and reliability (3.5 ulog for 100L samples in 90% of the cases) of the parasitic helminth eggs retention process that takes places on the filter cloth. Those results illustrate the safety of the reclaimed water obtained, with parasitic helminth egg concentrations 10 times lower than the limit recommended by WHO guidelines (2006) and that established by the Spanish RD 1620/2007 (1 egg/10L). In summary, the Hydrotech Discfilter functions as an effective and reliable barrier against helminth eggs. Its small footprint, its low energy consumption and its simple operation and maintenance requirements make it a competitive and attractive technology for water reclamation and reuse. The full study report has been published by Veolia and is available at http://www.veoliawaterst.es.