Biological fouling in water treatment systems

Microorganisms are present everywhere around us; in the air, in the water, in the earth and inside us and play a critical role in many biological processes. From a water treatment perspective, they are harnessed to break down organic carbon, ammonia and nitrate waste in biological water treatment systems. However, in membrane and ion exchange systems they are more of a problem and can lead to fouling and loss of water production. Many membrane systems (especially UF and MF membranes) have a primary function to remove bacteria and other harmful pathogens from drinking water.

What are the different types microorganisms?

A microorganism is a single-celled organism that can come in extremely diverse populations with high mutation rates that allows them to swiftly evolve and survive in new environments.

For the sake of discussion, viruses are generally regarded as not living and therefore not considered as microorganisms and are generally small enough that they won’t foul membrane systems. However, it’s important to understand virus removal in membrane systems when it comes to pathogens in drinking water.

Bacteria

Bacteria are ubiquitous (found everywhere), single celled organisms with no cell nucleus or other membrane-bound organelle. Bacteria function and reproduce as individual cells, but they can often aggregate in multicellular colonies. Bacteria are vital in many stages of the nutrient cycle including the fixation of nitrogen from the atmosphere.

Bacteria exhibit an extremely wide variety of metabolic types and can energy from both light (photosynthesis) and from breaking down chemical compounds (oxidation). These can then be divided into aerobic organisms that use oxygen, or anaerobic organisms use other compounds such as nitrate, sulphate, or carbon dioxide as an oxidation source. Different types of bacteria are used for BOD removal, denitrification, nitrification, sulphate reduction and anaerobic methane formation processes.

Many bacteria are motile (able to move themselves) and do so using a variety of mechanism including the use of flagella that are long filaments that are turned by a motor to generate propeller-like movement.

Aerobic bacterial: Aerobic bacteria survive and grow in an oxygenated environment and use oxygen to consume organic matter and convert it into carbon dioxide. Some examples are Nocardia, Pseudomonas aeruginosa, E. Coli, Citrobacter, Klebsiella, Salmonella. There are different levels of oxygen requirements; Obligate aerobes require oxygen to grow, facultative anaerobes use oxygen if it is available, but also have anaerobic methods of energy production.
Anaerobes: An anaerobe does not require oxygen for growth, and some even die if oxygen is present. Their key feature is that they use non-oxygen sources to digest their food including CO2, sulphate, nitrate and even metals like iron or manganese. Anaerobic bacteria cause many infections in the human body including food poisoning, soft-tissue infection and other infections. Examples include Bifidobacterium, Fusobacterium, Porphyromonas, Prevotella, Actinomyces,
Pseudomonas Aeruginosa: Pseudomonas bacteria are strict aerobes that are ubiquitous in water systems and are able to colonize a wide range of niches. A significant number of cells produce exopolysaccharides and biofilms which are difficult to remove from pipes and equipment surfaces. Pseudomonas is often responsible for the “fruity” odour of wet cloths and slimy films that develop in pipes. They are also widely responsible for biofouling in membrane systems. These dense biofilms can also host other bacterial species, including anaerobes such as legionella.
Iron Related Bacteria (IRB): Iron related bacteria (or iron oxidizing bacteria) are organisms that derive energy by oxidizing dissolved iron. They naturally occur in soil, shallow groundwater, and surface waters. These bacteria combine iron (or manganese) and oxygen to form deposits containing iron oxide, bacterial cells, and EPS slime. When de-oxygenated water reaches a source of oxygen, iron bacteria convert dissolved iron into an insoluble reddish-brown gelatinous slime that discolours water and equipment. Organic material dissolved in water is often the underlying cause of an IRB population. IRB can also oxidize manganese to form black manganese dioxide but is less common because of the relative abundance of iron (5.4%) in comparison to manganese (0.1%) in average soils. The sulphurous smell of rot or decay sometimes associated with iron-oxidizing bacteria results from the enzymatic conversion of soil sulphates to volatile hydrogen sulphide as an alternative source of oxygen in anaerobic water.
Sulphate Reducing Bacteria: Sulphate-reducing bacteria area group that can perform anaerobic respiration utilizing sulphate and reduce it to hydrogen sulphide (H₂S) rather than oxygen. Most sulphate-reducing microorganisms can also reduce some other oxidized inorganic sulphur compounds, such as sulphite, thiosulfate, polysulfides and elemental sulphur. Most of them are anaerobes; however, there are examples of sulphate-reducing microorganisms that are tolerant of oxygen, and some of them can even perform aerobic respiration.
Coli and Coliforms: Coliform bacteria and e. Coli are bacteria that are associated with the digestive system of warm-blooded animals. They can be aerobes or facultative aerobes. Due to the high concentrations in animal and human faeces and the fact that cells are only able to survive outside the body for a limited amount of time, these bacteria are used as an indicator for faecal contamination of foods, milk, and water. Since there are numerous naturally occurring coliforms they are used as an indicator of water contamination. However, e. Coli is generally only associated with intestinal systems and thus used as a specific indicator of faecal contamination (however as always there are exceptions).

Algae

Algae is a term for a large and diverse group of photosynthetic organisms. Most are aquatic and autotrophic (they generate food internally). The most common definition of algae is that they have photosynthetic machinery and are pigmented with chlorophyll. Seaweeds are actually one of the largest and most complex algae species.

Algae as a particular challenge in water treatment systems because of their ability to form in very large numbers in natural waterways. Both algal cells and excretions (EPS) can foul membranes and ion exchange systems. Another particular trait is that algal colonies can adsorb toxic materials including mercury, lead, arsenic, chromium, cadmium, manganese, iron, as well as pesticides and other organic pollutants which can enter drinking water or foul water treatment systems.

Viruses

Viruses are the most numerous types of microbe on the earth and are a microscopic infectious agent that replicates only inside the living cells. Most virus species are one-hundredth the size of bacteria and too small to be seen with an optical microscope.

Viruses infect all life forms, from animals and plants to microorganisms, including bacteria. When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell, viruses exist in the form of independent particles, which are protected by a coating of proteins or lipids.

Higher Order Microbes (Rotifiers and Protozoans)

Higher order microbes play an important role in processing waste and break down both larger suspended organic solids as well as consuming bacteria and dead bacterial cells. The protozoans in fall into four major classes: amoebae, flagellates, and ciliates (free-swimming, crawling, and stalked).

Amoebae: Amoebae are the most primitive, single-celled protozoans. They move by false feet and consume suspended solids. They can only multiply when there is an abundance of nutrients as they move very slowly, and it is difficult for them to compete for food.
Flagellates: Most flagellates absorb dissolved nutrients and are present when there are high concentrations of soluble food. Flagellates and bacteria both feed on organic nutrients water so as the nutrient level declines they have difficulty out competing the bacteria for soluble food.
Ciliates: Ciliates are higher microbes that primarily feed on bacteria and bacterial cells instead of dissolved organics.
Rotifers: Rotifers are rarely found in large numbers in wastewater treatment processes. The principal role of rotifers is the removal of bacteria and contribute to the removal of effluent turbidity by removing non-flocculated bacteria.

Microorganisms and fouling of membranes.

Microorganisms can cause a wide variety of problems in water systems and a lot of effort is put into controlling them. However, their ubiquitous nature makes elimination is almost impossible. The first way that microbes (and especially bacteria) impact a system is from their cells. Free-floating (or planktonic) bacteria can deposit on membrane surfaces and grow there. These cells will appear as suspended solids and can reduce flow rates. The second and more common issue, is with substances that microbes excrete. Most microbes will excrete proteins and polysaccharides that are termed Excreted Poly Saccharides (EPS) as part of their normal life cycle and when they wish to form protected colonies. They also start excreting more of this to protect themselves from harsh chemicals and when under stressed-conditions like when nutrients are in short-supply, and they start to starve. These are the well-known smelly bio films slimes that form on water treatment equipment and can range from a few micrometres in thickness to up to half a metre in depth, and may contain multiple species of bacteria, protea and archaea.

Understanding and controlling this EPS production is very important for controlling fouling.

Image copyright American Chemical Society

Biofouling Mechanisms

Membrane biofouling can be the result of aerobic bacteria, such as those found in brackish and seawater systems, as well as wastewater, and/or anaerobic bacteria in ground water systems, such as iron- and sulphate-reducing bacteria.

Biofouling begins with the formation of a conditioning film. A conditioning film is comprised of adsorbed organic and suspended materials that collect on the surface of the membrane. Organic compounds that will promote the growth of microorganisms include carboxylic and amino acids, proteins, and carbohydrates. Concentrations as low as parts per billion (ppb) of these organic compounds can lead to significant biofouling-based clogging of spiral-wound element feed channels. Higher water flux through the membrane brings new compounds to the membrane surface, thereby increasing the rate of accumulation in the concentration polarization layer, thus favouring the formation of the conditioning film. At this point is biofouling development, microorganisms are still reversibly attached to the membrane.

The next steps in membrane biofouling are adhesion of the bacteria and cell growth with microcolony formation. Higher concentrations of microorganisms coupled with quicker transport to the membrane surface (as a function of water flux) can serve to exacerbate the accumulation of microorganisms and the rate of bacterial adhesion. Adhesion enables bacterial cell growth and the formation of microcolonies.

Extracellular polymeric substances (EPS) are formed as growth and the formation of biofilm matures. EPS consist primarily of polysaccharides, proteins, glycoproteins, and lipoproteins. EPS serve to protect the bacteria from biocides, flow shear, and predators. About 90% of the resultant biofouling structure is composed of EPS, with the remaining 10% microorganisms.

The final step of membrane biofouling is the plateau phase. This is an equilibrium phase where attachment is essentially in equilibrium with detachment that occurs due to fluid shear forces. In a membrane module and in the membrane system itself, this phase is critical to the proliferation of bacteria and resultant biofouling further along the feed channel, thereby expanding the degree of membrane surface area infected up to including the entire system.

Detecting bacterial fouling

Biofouling can be indicated in a membrane system by a loss of flux (as the bacteria and biofilm coat the membrane surface) with no associated loss of salt rejection that would be indicative of scale formation. Often there is a significant increase in the DP of the system as the bulky biofilms block the feed channel and feed spacer in the membrane.

Testing for bio formation can be done by measuring organic carbon (TOC or DOC) in the feed water and concentrate. This will give a measure of both food organic carbon and also organic materials associated with the bacteria or microbes.

Bacterial activity tests such as heterotrophic plate count (HPC) are used to measure the amount of active live bacteria present and there is a wide range of specific tests for individual microbial species available.

Managing microbial fouling

The first step of managing microbial fouling is to track and identify the source. This can be done by using dip-slides or taking samples across water sources and the plant. If the source can be managed this is great, if not then futher work must be done. Algae can be treated using a variety of techniques including coagulation, copper sulphate treatment, ultrasonic treatment or sterilization. Bacteria can usually be treated by managing their food sources or by sterilization using oxidizing (chlorine, hypochlorite, chloramine, UV) or non-oxidizing biocides (DBNPA is the most common). Of note is that it’s almost impossible to have a sterile system and microbes will generally always re-infect when there is a food source available. One approach is to use a biofilter (a wetland or bacterially active sand filter) to adsorb and react away nutrients that are being consumed by the bacteria, if their food source is reduced or eliminated then growth will be significantly decreased. In microbial active water dosing of DBNPA into your membrane is recommended.

Other membrane friendly biocides like peracetic acid of peroxide can be used but your membrane manufacturer should be consulted to understand compatibility risks.

The final way of managing microbial fouling is through cleaning. There are a number of cleaning options on the market. The most common is to use a high pH caustic cleaner (NaOH pH 12 35C) which will hydrolyse the biofilm and hopefully remove it. If this isn’t effective then more complex cleaning agents may be necessary, surfactant cleaning can emulsify and remove biofilm as can specialized enzymatic cleaners.

Do you have a biofouling issue? Contact us today to discuss how we can help you get back on line quickly.