LuminUltra – Industrial Water
Monitoring Biological Growth in Membrane Filters
Membrane filtration units are widely used in the water treatment industry for applications ranging from groundwater treatment and desalination to industrial water treatment and wastewater processes. While they are capable of producing extremely high-quality water, they require a significant investment to acquire and maintain therefore it is important to protect the membranes from excessive wear in order to maximize the lifespan of the modules.
Due to the porous nature of the membranes, they are susceptible to both chemical scaling and biological fouling which can increase energy requirements and operating costs, and shorten their useful lifespan. Treatments differ depending on the nature of the fouling and each cleaning cycle imposes extra stress on the membranes so cleaning chemicals should be targeted to the specific type of fouling that is present and cleaning frequency should be optimised.
Through adenosine triphosphate (ATP) testing, influent water quality can be monitored; permeate water quality can be assessed to reveal contamination and microbiological break-through; and the membrane fouling can be assessed. By monitoring membrane fouling on a regular basis, the appropriate treatment can be applied and cleaning cycles can be optimized to reduce excess wear on the membranes.
Calculating the Membrane Fouling Index
To calculate the membrane fouling index, ATP concentrations (pg/mL) are tested in the influent, permeate, and concentrate or reject streams. The concentration of ATP is then calculated for each based on the respective flow rates (Q).
Mass Balance: (Qinfluent x ATPinfluent) = (Qpermeate x ATPpermeate) + (QConcentrate x ATPConcentrate)
Initially, the amount of biological content entering should be equal to the amount leaving the system in the permeate and concentrate with a significantly greater portion in the concentrate and very little break-through in the permeate. Through normal operation, the membranes will wear and microorganisms and solids will build up on the surfaces which can lead to greater concentrations in the concentrate and potentially in the permeate. The fouling index, a ratio of the amount of ATP leaving the system vs. the amount entering, allows operators to quantify the amount of biological contamination that has built up inside the system.
Fouling Index (FI) = [(Qpermeate x ATPpermeate) + (QConcentrate x ATPConcentrate)] / [(Qinfluent x ATPinfluent)]
- If the fouling index is less than or equal to 1, the membrane is not experiencing significant biofouling.
- As the fouling index increases beyond 1, the more heavily it is biofouled.
Systems can be quickly audited on the spot to determine the appropriate treatment while regular testing allows utilities to monitor how changes in the influent affect membrane fouling and permeate quality.
ATP testing provides membrane filter operators the ability to quickly assess biological growth anywhere in the system. With regular monitoring, ATP baselines can be set and cleaning schedules can be optimized to reduce energy requirements, optimize chemical use, and prolong the lifespan of membrane modules.