Effect of sludge treatment processes on ASCARIS destruction for safe utilisation of sewage sludge in Egypt

Abstract

Two different types of sludges from two popular sewage treatment systems were used in this study; namely, anaerobic pond sludge from a wastewater stabilisation pond system and filter pressed primary sludge from a primary sewage treatment plant. Under Egyptian conditions, Ascaris eggs represent the main health concern for sludge application on land and, accordingly, the effect of sand drying beds and a passive composting system (using agricultural wastes as bulking agents and cement dust as an additive to the sludge) were studied in terms of the inactivation of Ascaris eggs. From the characterisation and assessment of the anaerobic pond sludge from the Mit Mazah wastewater stabilisation pond system, it has been concluded that the sludge was well digested and free of viable Ascaris eggs. Consequently, this sludge is considered to be safe for land application, after appropriate dewatering. For the purposes of the study, anaerobic pond sludge was seeded with Ascaris vitilorum eggs (cattle ascarid). Within 30 days of dewatering on sand drying beds during the winter season (October and November), the Ascaris eggs were found to be inactivated only in the top layers, where the average solids content was approximately 85%. By the end of the two month drying period, the middle and bottom layers of sludge on the drying beds contained an average solids content of 74% to 77% and 72 to 73%, respectively, yet viable Ascaris eggs were still detected. During a second sludge application during the winter season (December and January), unfavourable weather conditions prevailed and, by the end of the 60 days drying period, total solids content averaged 54% in the top layer of sludge. The middle and bottom layers had an average total solids content of 49% and 44%, respectively. Throughout the winter drying period, all samples collected were positive for viable Ascaris eggs throughout the full depth of sludge. However, during the summer season, samples collected from the drying beds after 17 days of drying did not contain viable Ascaris eggs in the top sludge layers, where the solids content ranged from 74% to 63%. After one month of summer drying, no viable Ascaris eggs were detected throughout the entire depth of sludge on the drying beds and the lowest solids content recorded was 80% for the bottom layers. From this study, it can be concluded that, during the wet season in Egypt, drying beds were not efficient in destroying Ascaris eggs throughout the entire sludge depth. During the summer season, Ascaris eggs can be inactivated much more rapidly, compared to the winter season. Desiccation does not seem to be the only factor influencing the destruction of Ascaris eggs, with sludge temperature, solar radiation intensity and exposure time being possible major factors. By comparing the composting of filter pressed primary sludge in passive and windrow piles (used as control piles), it has been proved that the passive composting system is much more efficient in maintaining regular high temperatures in the composting mixture for a much longer duration (additional 10 days), due to the favourable aerobic conditions. Moreover, passive composting preserves the nitrogen content through the naturally controlled internal temperature, less need for turning and lower loss of moisture. By the end of the composting process, the total nitrogen content for the passive and windrow piles was a gain of 17.5% and a loss of 0.7%, respectively, while the ammoniacal nitrogen content in the product was 1109 mg/l and 837 m/l, respectively. From the hygienic point of view, passive composting technology proved to be much more effective in inactivating Ascaris eggs present in the primary sludge, compared to the windrow pile system. Viable Ascaris eggs were not detected after 30 days of composting using the passive system, compared with 60 days of composting for the windrow pile. Industrial cement dust as an additive and agricultural wastes (from fennel and basil production) were incorporated with sludge to form a range of composing mixtures. From the sludge/cement dust composting piles with agricultural wastes as bulking agent, it can be concluded that more than 35% concentration of cement dust was unfavourable for the decomposition process, due to the high pH and high temperature levels attained as a result of the effect of cement dust and dehydration of the organic matter. Moreover, loss of nitrogen was very high, due to the release of ammonia, encouraged by the high pH and high temperature in the composting mixture. The passive composting system was very efficient in naturally controlling the internal temperature in all the sludge/cement dust piles with bulking agent. Excessive heat was released to the atmosphere through the chimney effect created by the circulation of air through the perforated pipes and upwards into the pile. Internal temperatures for even the piles with the highest proportion of cement dust did not exceed 73°C. Inactivation of Ascaris eggs was achieved after only 15 days of composting for all the different cement dust concentrations, mainly due to the influence of high pH and high temperature levels for long periods but may also be due to the high release of ammonia gas. The passive composting of sludge amended with 30% cement dust without bulking agent was inefficient for both the decomposition process and inactivation of Ascaris eggs. No biological activity was taking place during the two months fermentation stage, due to the prevailing anaerobic conditions (lack of air voids and high moisture content) By considering all the different parameters for evaluating a treated compost, including the stability, maturity, nutrient content, heavy metals content and viable Ascaris eggs content for the sludge/cement dust piles with bulking agent, it can be concluded that more than 30% cement dust concentration would be unfavourable, according to the relatively high loss of nitrogen. More than 35% cement dust concentration is certainly unfavourable, mainly as a result of the delay of the decomposition process, the elongation of the composting period and the high loss of nitrogen content. There is no need to separate out the agricultural waste bulking materials after composting because they are organic in nature. Likewise, there is no need to consider separating out the cement dust, as the physicochemical analysis of the final sludge/cement compost with bulking materials showed its suitability for adding to agricultural land as an amendment. In conclusion, sand bed drying is considered to be a suitable process for dewatering of anaerobic stabilisation pond sludge in Egypt and, for extended periods between pond desludging of the order of 6 years, the dewatered sludge will be suitable for direct application to agricultural land. Passive composting is an appropriate technique for preparation of Egyptian filter pressed primary sludge for safe application to agricultural land. Cement dust additions of less than 30% concentration, to the composting mixture in a passive composting system produces sanitised and mature compost. The potential for co-disposal of agricultural wastes and cement kiln dust as bulking agents and additive, respectively, with sewage sludge in the production of compost, is very promising to produce a safe and beneficial outcome to the community.