MBR-RO
- 1. Overview
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Submerged MBR technology has successfully been used as the pretreatment option for challenging industrial
and municipal water reuse RO systems, and can help make water recycling technologies more cost-effective.
While extremely effective on biologically treated wastewater, RO systems need to be coupled with an effective
pretreatment system to avoid common issues that can result in system failure, including plugging, fouling and scaling.
A pretreatment option for wastewater applications is membrane bioreactor (MBR) technology,
in which a membrane process like ultrafiltration (UF) or microfiltration is combined with a suspended growth bioreactor.
MBR provides high quality feed water to the RO, minimizes footprint and the cost of civil works,
and reduces treatment plant downtime, thereby reducing operating costs.
MBR-RO Systems are widely used in wastewater reclamation applications.
- 2. RO a Challenge for Reuse
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Pretreatment methods are critical when designing RO systems. For example, RO membranes used for most water
reuse applications contain a brine spacer, typically made of low density polyethylene mesh netting.
If there's a high level of suspended solids in the feed water, this brine spacer can become plugged.
Another issue is the high levels of organics contained in many biologically treated wastewaters,
which are rejected by the RO membrane and progressively concentrated as the water flows across the membranes.
This concentration of organics can foul the membrane, especially towards the RO system outlet.
Biofouling can also occur, because the organics in wastewater make an excellent food source for microorganisms.
And some treated wastewaters contain high levels of bacteria, so biogrowth may occur quickly even if RO feed water is disinfected.
Finally, calcium phosphate scaling can cause problems with RO systems operating on some wastewaters.
This scaling can be mitigated by operating at lower water recovery, using acid or other antiscalant to minimize scaling,
or modifying the operating conditions of the WWTP to reduce the amount of phosphate in the RO feed.
These plugging, fouling and scaling issues mean the RO system needs to be operated at higher pressures,
leading to increased power consumption, higher chemical costs for cleaning, and a shorter membrane life.
How can these challenges be minimized and overall water reuse system lifecycle costs reduced?
Effective pretreatment of the feed water before it flows through the RO system is the answer,
provided the pretreatment steps are chosen carefully to ensure the RO system can work as intended.
- 3. RO Pretreatment Options
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There are many different pretreatment options, and the best for a particular process depends on power,
chemical, labor and land costs, wastewater source, and the existing wastewater treatment system.
3-1 Conventional pretreatment -The conventional effluent pretreatment scheme might be primary treatment,
biological treatment and, the most crucial part of the process, solids-liquid separation using secondary clarification.
A conventional sedimentation process often doesn't remove enough bacteria and suspended solids,
so sand filtration may be added to improve solids-liquid separation and provide higher quality water to feed the RO system.
3-2 Using Ferric Chloride along with Sand Filtration may enhance solids and organics removal.
Upsets in the secondary clarifier, however, can lead to effluent with higher levels of TSS and BOD,
causing plugging of the brine spacer with suspended solids and organic fouling.
Power consumption for RO systems with this type of pretreatment tends to be high, and membrane life is often quite short.
3-3 Lime Softening has been somewhat more successful in protecting the RO membranes,
but this increases operating costs and doesn't totally prevent RO membrane fouling.
3-4 UF improves TSS removal -- Many of today's water reuse systems use an ultrafiltration (UF)
pretreatment step to remove suspended solids. These systems typically use hollow fiber UF membranes,
which do an excellent job of providing water with low suspended solids to feed the RO system.
Still, the UF system is an extra treatment step, requiring additional footprint and adding to operating costs.
The UF system may also be susceptible to upsets from a conventional WWTP, which can further increase its operating costs.
3-5 MBR as RO pretreatment - With an MBR, the UF membranes are submerged in the activated sludge to combine the biological
step and solid-liquid separation into a single process. The membrane acts as a barrier, which improves the effluent quality.
The MBR eliminates the secondary clarifier and doesn't rely on gravity for liquid-solids separation,
which allows the activated sludge to operate with a higher mixed liquor suspended solids (MLSS) concentration.
An increased MLSS concentration reduces bioreactor tank volume, saving footprint and capital construction costs.
Overall, the MBR process reduces footprint significantly compared to the combination
of conventional activated sludge followed by sand filtration or UF.
The footprint savings due to the wastewater treatment plant alone can be as much as 50%,
along with additional footprint savings from eliminating other filtration steps.
Using MBR technology also simplifies the overall treatment train, minimizing the number of unit operations.
- 4 MBR-RO Systems
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4-1 Major components
· Cartridge / bag filter
· Pressure vessels with spiral wound membrane elements
· High pressure pump
· Control and instrumentation with fail-safe devices
4-2 Cleaning-In-Place (CIP) Systems
· Cleaning pump
· Cleaning tank
· Bag filter
· Control and instrumentation
4-3 Post-treatment
· Chemical dosing system
· Neutralisation
· Chlorination (Calcium / Sodium Hypochlorite)
2. Process
Depending on the feed water quality, MBR-RO Systems usually requires some pre-treatment systems such as:
Feed pump, Chemical dosing systems, disinfectant, acid and anti-scalant dosing.
