Reuse of Municipal Wastewater for Industrial Plant Supply
A summary of the paper titled
“Reuse of Municipal Wastewater as Industrial Plant Supply –
Pilot Testing for Full Scale System Design,”
presented by Jason Stevens, Vertical Market Manager – CPI,
at the International Water Conference 2016
As large industrial plants look to sustainably grow their production in the context of water
scarcity, they are increasingly looking at options such as reuse of municipal wastewater
effluent to meet some or all of their increasing demand for fresh water make-up. In some
areas, treated wastewater from multiple plants is carried in a single pipeline to multiple
potential users. This blending of waters minimizes the amount of change in the water
based on upsets or variations in operations at a single wastewater treatment plant, and
allows industrial users to base equipment design and operations upon similar systems
operating on the same water. However, in many locales treated wastewater is only
available from individual treatment plants, adding a level of complexity to the reuse
process. Evoqua Water Technologies works with industries to identify, plan for, and
overcome difficult water treatment challenges, including wastewater reuse.
A large chemical plant along the Gulf Coast decided to use municipal wastewater from a
nearby city to supply a significant portion of the cooling water and boiler make-up water
required for a future plant expansion. This decision was influenced by several factors,
including periodic droughts, lack of senior water rights to their primary source of fresh
water, salinity of site well water, and a desire to broaden options for sustainable fresh
water make-up. Initial analyses of the municipal wastewater effluent indicated moderate
levels of suspended solids, organics, phosphate, chloride, and reactive silica. Based on
anticipated variability in the feed water, as well as quality and space considerations, a
preliminary system design was developed for the plant using ultrafiltration (UF) for
suspended solids removal followed by double pass reverse osmosis (RO) for dissolved
solids, silica, and organic carbon removal.
Bench scale testing confirmed that the rate of increase in trans-membrane pressure (TMP)
was acceptable at moderate flux and with low levels of coagulant. Recovery of membrane
permeability using only mechanical backwashing was very good, even after several
filtration cycles.
Based on the successful bench results, the plant decided to proceed with long-term pilot
testing of a UF and RO system at the municipal treatment plant. The pilot protocol was
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designed to gather data on the variability of the feed water and the quality of water
produced by the UF and the RO and also determine the maximum and optimum flux and
recovery on the UF and RO membranes based on chemical consumption and rates of
fouling. Key variables such as temperature, turbidity, flow, pressure, conductivity, and pH
were continuously monitored at appropriate locations and recorded using Evoqua’s
Link2Site® remote monitoring system. UF operating conditions were logged and
documented in an electronic workbook. RO operating conditions and water quality were
logged and entered into Evoqua’s RO normalization program for monitoring and
evaluation. With these parameters known, the capital and operating costs of a full-scale
system could accurately be determined.
Evoqua performed a turn-key installation of the pilot system at the municipal wastewater
treatment plant, including installation of power services and a lift pump from the
wastewater effluent canal and tie-ins to the plant’s feed and outfall for the waters produced
by the pilot. The pilot system operated for twelve continuous months allowing the project
team to evaluate the effects of a wide range of feed water conditions based on seasonal
variations, short-term weather events, and wastewater treatment plant upsets. Time also
permitted testing of several different operating parameters for the equipment, including
coagulant dose, and flux and recovery in both the UF and RO units. Weekly operational
reviews were conducted between Evoqua and the customer in order to review the testing
progress, current operational status, system maintenance, and future actions.
The pilot testing revealed strong fluctuations in wastewater turbidity, conductivity,
phosphate, nitrate, and total chlorine levels, as well as smaller variations in pH, TOC, silica,
and ammonia. The UF and RO systems exceeded the stated recovery and quality objectives
of the testing, allowing for these operations to be smaller in the full scale design. At the
same time, important data were gathered regarding the best operating conditions and
plans were formed for addition of support systems to allow the process equipment to
function more reliably and with less operator involvement.
The UF system operated successfully at a higher flux and recovery than anticipated, even
during high turbidity events in the incoming water.
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Reduction in coagulant dosage resulted in rapid TMP rise across the UF, but the system was
able to recover quickly when coagulant dosage was increased. Reduction in phosphate and
TOC across the UF were at the low end of the expected range and did not improve as much
as expected with increasing coagulant dosage rate. After nine months of operation, the UF
experienced a fouling event that required several consecutive CEB’s to recover from.
Outside of this event, regularly scheduled CEB’s were able to restore normalized
permeability to baseline conditions.
The RO system operated well at the planned flux and recovery, but was not able to be run
more aggressively due to fouling concerns. Chloride, silica and other key constituents were
continuously removed to levels below the desired thresholds. RO uptime was greater than
95% over the course of the twelve month pilot. As expected, dechlorination of the RO feed
allowed for microbial growth in the RO. Combined with moderate to high levels of
phosphate and TOC, this required that the RO membranes be cleaned every 2 months on
average, based on the manufacturer’s normalized differential pressure recommendations.
Autopsies of the lead and tail RO membranes showed that the bulk of the foulant was
organic in nature, with some calcium phosphate present.
Based on these results, Evoqua made several recommendations for the design of the future
full-scale system: addition of enhanced coagulation ahead of UF to increase removal of
organics and phosphates; online monitoring of TOC and phosphate concentrations to allow
for automation of the coagulation system; reduction in UF system size, backwash duration,
and CEB frequency from the preliminary design values; maintaining the RO system flux and
recovery; adding a permeate flush to the RO system; inclusion of provisions to add a
biocide feed to the RO; and a robust clean in place system to restore UF and RO membrane
performance after a fouling event.
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Overall, the pilot testing demonstrated that the municipal wastewater source is a feasible
alternative to the current water source. The primary performance targets with respect to
system recovery and permeate quality are achievable in a long term operation even with
variable feed water quality.
The full version of this whitepaper is available for purchase and download at:
https://eswp.com/water/conference-archives/proceedingspapers-order-form/
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