This study set out to investigate the processes that lead to emitter fouling in wastewater reuse systems. The susceptibilities of several emitter designs to clogging were compared and the role of phytoplankton in the clogging process and in the development of algal mats was investigated. Emitter design was found to be an important factor controlling the degree of clogging. Those designs that operated most efficiently used a long-path narrow labyrinth to control emitter output and encourage turbulent flow. Simpler designs that controlled flow by small sponge inserts or by stitched tubing were found to be unsuitable for use with treated wastewaters. The most common cause of clogging was found to be sand particles in the size range 360 urn to 1080 urn, trapped within the narrow channels of emitters. The sand contaminated the entire irrigation system from the potable water supply in July 1987, before a screen filter was installed in the water supply line. Emitter clogging under these circumstances was a problem, to varying degrees, for all emitter designs and for all water qualities. Emitters supplied with WSP effluent generally clogged to a greater degree than those supplied with potable water and the principal cause of clogging was shown to be sand particles. However, no statistically significant difference was recorded in the discharge characteristics of the most efficient emitter design between different water qualities. Thorough cleaning of the irrigation laterals in 1987 and replacement of emitters in 1988 failed to eradicate the sand contamination problem. Examination of clogged emitters by electron microscopy showed that organic material encased the sand particles in clogged emitters from laterals supplied with WSP effluent, thereby sealing the water channel. This mass was shown to be comprised of dead microalgae and invertebrate animals such as Daphnia spp. on which bacteria developed. Microalgae did not multiply in the dark environment of the emitter interiors. External algal mats were detected on less than 5 percent all emitters supplied with WSP effluent and were absent on all emitters supplied with potable water. Emitter C, which was the design that presented the largest wetted surface to sunlight, developed the greatest number of mats and covering these emitters with black polythene prevented mat development. The mats were shown to comprise of predominantly filamentous cyanobacteria (Oscil/atoria spp. and Lyngbya spp.) and filamentous green algae (Microcystis spp.). Although these organisms were also detected on the walls of the maturation pond, they were not detected in grab samples of pond effluent which contained predominantly planktonic algal genera such as Euglena spp. and Chlorella spp. Short decaying filaments of Oscillatoria spp. were, however, detected within the irrigation laterals and on the surfaces of sand particles within clogged emitters and no degree of filtration would guarantee their complete removal from the pond effluent. It was hypothesised that their development on the outer surfaces of emitters was a result of colonization of a well-illuminated, wet and nutrient-rich environment and that the source of the inoculum was as likely to be the soil as to be the maturation pond. Studies of greenhouse irrigation systems in the UK revealed that algal mats were a consequence of applying inorganic nutrients to the crop in the irrigation water: their development can be prevented in greenhouses by chlorination of the water supply. It was concluded that clogging results from a combination of physical, chemical and biological factors. Sand particles can be efficiently removed from the water supply by incorporating a simple screen filter, with a mesh size of at least 120, that allows microalgae to pass through the system to the soil. Clogging by sand particles was exacerbated by chemical precipitation of calcium carbonate and by the development of a organic material over the surface of the sand particles in emitters supplied with pond effluent. Biological growth alone was not shown to cause emitter clogging. It was also shown that the development of algal mats over the outer surface of emitters supplied with pond effluent were not a result of an accumulation of pond algae at this point but represented an opportunistic colonization of a well illuminated nutrient-rich environment by microorganisms from the atmosphere and/or soil. Algal mats had no adverse effect on the operation of emitters in WSP reuse irrigation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:317267 |
| Date | January 1992 |
| Creators | Taylor, Huw David |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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