Microbial fouling of drip irrigation equipment in wastewater reuse systems.
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
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
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.