As utilities develop facilities to maximize water resources, indirect potable reuse and direct potable reuse are becoming more common.

Microfiltration or ultrafiltration membranes are often installed for pretreatment ahead of reverse osmosis, and advanced biological treatment often precedes membrane treatment.

However, the integration of advanced biological treatment and tertiary membrane treatment result in operations and maintenance challenges. When the two treatment worlds collide, it’s helpful for operators to understand the issues and their options to keep both processes working effectively and consistently.

Daily disagreement

Biological and membrane process flows need fundamentally different management. Membranes want a steady influent flow, whereas biological systems have to treat flow as it arrives.

Membranes, especially RO, typically do not treat highly variable diurnal flows and are more difficult to operate under such flow conditions.

Figure 1 illustrates a typical diurnal flow pattern for a wastewater treatment plant with a 10 mgd average flow. Hourly flows can vary by as much as 30 to -60 percent. In such a plant, membranes will experience significant flow variations throughout the day. Although MF and UF membranes can typically handle flow variations, RO systems have a more limited range — from 85 to 100 percent of their design flow.

So for the plant just described, the RO membranes could not process more than 10 mgd of influent, and the RO trains would have to be turned off when influent flow dipped below 8.5 mgd. As a result, a significant fraction of effluent could not be reused. The RO trains would need to be taken out of service and flushed or cleaned daily, since biological growth tends to accumulate on idle RO membranes.

Alternative solutions

The obvious solution to this problem is flow equalization. There are multiple equalization options each with advantages and disadvantages. Inline equalization often requires separate pumping of the forward flow, whereas offline systems can store just the flow in excess of the targeted continuous daily flow.

Option 1: Equalization ahead of RO

Equalization ahead of the RO system provides the flow relief needed to dampen hourly fluctuations and allows the RO to operate at a constant flow for longer periods. The RO system in this case can be designed for the true average daily flow rather than oversized to meet a peak hourly flow, thereby reducing the installed cost. Equalization also minimizes the number of times operators have to shut down, flush, clean the RO train and put it in storage mode until the flow rises.

While this simplifies operation of the RO flow equalization, it leaves the upstream facilities exposed to all daily and hourly flow fluctuations and the attendant operating complexity. The upstream facilities, including the MF/UF system, must be designed for the variable flows.

Option 2: Equalization ahead of MF/UF

Equalization ahead of the MF/UF system requires storage of high-quality biologically treated effluent containing low concentrations of suspended solids with little chance of creating odors.

A continuous or scheduled program of chlorine dosing is needed to prevent excessive slime growth. Even with equalization to dampen the diurnal fluctuations through the MF/UF, an RO equalization tank is still needed to simplify RO operation.

As with Option 1, this equalization option allows the MF/UF system to be designed for lower, longer-term flow averages, so it can be operated at a more steady state for longer periods, simplifying operation. In addition, chemical feed systems can be operated at steady state. Equalization lets operators focus more time on the big picture and less time tweaking and balancing chemical feed rates to address changing conditions.

Option 3: Primary effluent equalization

The biological plant can be a mystery for many membrane system operators. Equalization ahead of the biological process allows loading to be controlled throughout the day and minimizes wide load fluctuations. Although equalization of flow matters most for membrane operations, equalization of load matters more for biological systems.

The BOD and TKN loads drive aeration and sludge wasting, which affects the need to adjust airflow rates for dissolved oxygen control, MLSS recycle rates for denitrification, and return/waste activated sludge flow rates. Managing the load produces steadier and simpler daily operation. A steady daily flow also simplifies clarifier operation. This option enables operators to better control the biological process and so produce a better-quality effluent with less colloidal material to feed the membrane systems.

Equalization also improves biological system performance by increasing removal of soluble BOD, thereby limiting RO membrane fouling. Less frequent major cleaning of the membranes extends membrane life and also cuts operating costs by enabling operation at a lower pressure.

Equalization of load before biological treatment has a downside — odor generation. In effect, the utility pays for less frequent membrane cleaning with cleaning and odor management of the equalization tank. As a remedy, two equalization tanks provide operating flexibility and make tank cleaning and maintenance easier.

Overall, primary effluent equalization is better than Options 1 and 2 because it produces a better-quality feed to the membranes and simplifies operation of three major sections of the facility. On the other hand, integration of primary effluent equalization into existing facilities considering potable reuse can be challenging where there are significant site constraints.

Option 4: Influent flow equalization

Influent flow equalization can take many forms, including temporary storage in the collections system or an on-site tank. Storage of incoming flow is a major source of odors. Site constraints often dictate the choice between influent or primary effluent equalization, but primary clarifier effluent storage is generally preferred. Options 3 or 4 are generally better than Options 1 and 2 because they equalize flow and simplify biological and membrane system operations, while also yielding substantial improvements in biological effluent quality.

Sidestream equalization

The sidestreams from sludge digestion and dewatering have high concentrations of pollutants, especially in plants with anaerobic digestion. If these high-strength waste streams are sent back to the head of the plant, the influent or primary effluent equalization basins can catch and equalize the loads.

Because many plants dewater solids on an intermittent schedule, separate equalization and even treatment of this sidestream load can be beneficial to overall biological plant operation.

Reducing the sidestream loads simplifies operation of the main liquid stream process and produces a more stable effluent quality.

Polymers are used both to dewater and thicken sludges. While polymers are necessary, any polymer in the plant effluent increases the potential for membrane fouling; MF/UF systems are particularly at risk. Whether RO equipment is at risk depends on the type of polymer used. Solvent-based polymers, for example, can be problematic; solvents that are not biodegradable can foul and even damage RO membranes.

In many reclaim cases, only a portion of plant effluent is sent to the membrane systems. If all sidestreams containing polymer are separated and returned to the treatment train that does not send effluent to the reclaim system, polymer fouling can be avoided. If polymers are used properly in thickening and dewatering, almost all polymer should be absorbed onto the solids.

Testing can help operators determine whether a particular polymer contributes to or causes membrane fouling. Some newer polymers are biodegradable. Sending sidestreams back to the biological system allows microbes to convert the polymer to biomass. On-site pilot testing helps ensure that all components of the polymer solution are biodegraded and nothing slips through that would foul the membranes. An on-site membrane pilot unit also brings opportunities to investigate other membranes and membrane-cleaning procedures.

Final analysis

Biological and membrane treatment are very different, but both can significantly benefit from flow equalization. Equalization as far upstream as possible makes operation easier for both systems and ultimately produces better-quality reclaimed water.

RO systems do not have much flexibility for turndown, so some equalization ahead of the RO is usually needed to simplify operation, reduce operation and maintenance costs, and maximize reclaimed water production. Equalization within reuse systems benefits both biological and membrane systems and produces a better final product.

About the authors

Ed Kobylinski is a senior wastewater process expert, Neil Massart is director of the operations technology group, Dr. Sandeep Sathyamoorthy is process and innovation leader, and Jonathan Loveland is global practice leader for alternative water supply, all in the water business of Black & Veatch.