Team:Western Canada/Demonstrate
Integration into Municipal Wastewater Treatment Systems
In addition to modeling activities, the dry lab team developed a practical approach to integrate our biofilms into existing wastewater treatment systems. Our research in this area focused on medium and large-scale municipal wastewater treatment plants, such as the Greenway Wastewater Treatment Plant which we visited as part of the project.
Conventional wastewater treatment typically consists of four main stages 1. Preliminary treatment is mechanical; it involves the use of equipment such as bar screens to remove debris, and grit chambers to allow the settling of large solids. Primary treatment tanks allow most of the remaining solids to settle out of the wastewater, while floating material is skimmed off the surface. Secondary treatment processes typically use biological or chemical processes to degrade organics and remove remaining suspended solids. Tertiary treatment, or advanced treatment, is applied if concerning contaminants that cannot be removed through secondary treatment processes are present 1. This includes eutrophying pollutants like nitrogen and phosphorus, or pollutants that pose a health risk such as heavy metals.
Biofilms have been used extensively in wastewater treatment for many years. Compared to physical and chemical treatment methods, biological methods of wastewater treatment can be lower cost, simpler to maintain, and in some cases have fewer detrimental effects on the environment 2. Biofilm-based treatment processes can be divided into two groups: suspended/dispersed growth processes, where biomass is suspended in a liquid, and attached growth processes, where biomass grows on a surface 2.
The activated sludge process, which is the most commonly used method of secondary treatment, is an example of a suspended growth process 2. Trickling filters and rotating biological contactors are examples of attached growth processes that are typically implemented as tertiary treatment 2. More recently, research into the use of integrated fixed film activated sludge (IFAS) systems has been capturing attention. These systems are also called hybrid biological reactors, because they combine the properties of suspended and attached growth processes through the use of biofilm carriers 3. The immobilization of micro-organisms in a biofilm carrier results in a higher biomass concentration, which can translate to faster degradation of contaminants 4. Other advantages over traditional methods include lower space requirements, lower hydraulic residence time, improved resilience to disruptions in the environment, higher biomass residence time, and simple operational requirements 2. The adhesion of micro-organisms to the PVA-gel also helps to reduce sloughing. This reduces sludge generation and the associated removal costs, which is the primary drawback to most attached growth processes5.
We propose the use of PVA-gel beads as biofilm carriers. This material has already been used for wastewater treatment and is often introduced immediately before or in combination with the secondary treatment phase 6. By this stage, around 80% of solids have been removed from the wastewater but the concentration of organics and nutrient content remain high enough to sustain micro-organisms. The beads can be incorporated into existing wastewater treatment systems regardless of whether secondary treatment is aerobic or anaerobic 5. They are insoluble in water and non-biodegradable 7. Their highly porous structure strongly promotes the growth of micro-organisms and slows the rate of sloughing (breaking-off) of biofilm. This reduces sludge generation and lowers costs associated with its removal. Experiments have found that E coli. formed extensive colonies on PVA-gel beads3.
The beads would be coated in the lab, then moved to the treatment plant. Further research must be conducted into the costs of growing the biofilms in a controlled setting and subsequently transporting the coated beads without allowing the micro-organisms to die.
Addressing Potential Challenges
A number of environmental factors influence the effectiveness of biofilm wastewater treatment processes. Biofilm growth efficiency is highest in a certain narrow temperature range, and can be greatly reduced if temperature varies outside of it 2. The optimal temperature for E. coli is 37 degrees Celsius. For treatment plants located in colder climates, temperatures for all or part of the year may be a significant barrier to using our biofilms. Potential alternatives to E. coli should be investigated. Changes in pH can result in the death of micro-organisms, which would slow biofilm growth considerably 2. pH is controlled in wastewater treatment, and as E. coli are well-suited to environments with a neutral pH, this should not present a challenge to using our biofilms. The velocity and turbulence of the wastewater will impact the extent of growth of our biofilms 2. PVA-gel beads are often placed in a tank and circulated using water currents. This turbulent environment could reduce the thickness of the biofilms relative to a case where the beads are kept sedentary and the flow is laminar and extremely slow (for example, the packed-bed filter setup described in the following section). As the beads circulate with the currents, however, their velocity relative to the water would be lower than if the beads were fixed. This is an advantage over other attached growth processes, as it will allow a higher degree of mixing without causing breakage of the biofilm. The adhesion of the micro-organisms to the porous beads should also prevent excessive breakage. Despite this, trials should be done to determine the maximum flow velocity that the beads can be subjected to.
Prototyping
The method with which the PVA-gel beads would be introduced into existing wastewater treatment systems depends on the processes used in each facility. In practice, PVA-gel beads are often placed in an open tank and circulated continuously through mixing6. For systems with anaerobic treatment, this step can be removed from secondary treatment, and implemented as an entirely separate phase. We decided to propose an alternative design that would eliminate the need for aeration, thereby reducing energy costs. As described in a paper by Pandey and Sarkar 5, PVA-gel beads were found to be effective at breaking down contaminants when used in a packed-bed filter. Similar to the trickling filter described in the previous section, the filter consists of a packed bed of media (i.e. the beads). As wastewater flows through the filter, a biofilm forms on the media. After sufficient biofilm development, the filter can be used for treatment. As our biofilms are to be grown in the lab, our filter could be immediately put in service after adding the coated beads. The biofilm degrades contaminants as water passes through the filter media.
A rough prototype (seen below) was first developed as a proof of concept and for potential use in the lab. Wastewater would be added in the open top of the filter, which would be partially packed with biofilm-coated beads. Glass beads were substituted for PVA-gel beads, as they were a more convenient size for the prototype and more easily accessible. Had we progressed further in the lab, this simple non-mixed batch reactor would have been used to test the performance of the biofilms. This would allow us to determine if any conversion of estradiol and diclofenac was achieved, however, the efficiency of the reaction would likely be lower than a reactor using PVA-gel beads; it can be assumed that the non-porous structure of the glass beads would be less effective at immobilizing micro-organisms, resulting in a lower biomass concentration and slower contaminant conversion.
We also developed a more refined prototype design using CAD software (image seen below) (will provide a photo when I have it). This design consists of a small shell similar in size to the previous model and would also be packed with biofilm-coated glass beads. The device can be 3D-printed, but we did not reach the stage in the project where the prototype was needed so this step was never carried out.
Figure 1: (left) CAD model of our prototype of a packed-bed biofilm reactor (right). Simple model constructed for potential use in the lab.
References
1. Pescod, M. B. Wastewater treatment and use in agriculture. (1992).
2. Sehar, S. & Naz, I. Role of the Biofilms in Wastewater Treatment. in Microbial Biofilms - Importance and Applications (IntechOpen, 2016). doi:10.5772/63499
3. Singh, N. K., Singh, J., Bhatia, A. & Kazmi, A. A. A pilot-scale study on PVA gel beads based integrated fixed film activated sludge (IFAS) plant for municipal wastewater treatment. Water Sci. Technol. 73, 113–123 (2016).
4. ZHANG, L., WU, W. & WANG, J. Immobilization of activated sludge using improved polyvinyl alcohol (PVA) gel. J. Environ. Sci. 19, 1293–1297 (2007).
5. Pandey, S. & Sarkar, S. Anaerobic treatment of wastewater using a two-stage packed-bed reactor containing polyvinyl alcohol gel beads as biofilm carrier. J. Environ. Chem. Eng. 5, 1575–1585 (2017).
6. Kuraray Co. ltd. PVA-gel beads for immobilization of microorganisms.
7. Kuraray Co. ltd. PVA GEL: Polyvinyl Alcohol Hydrogel - A carrier for immobilization of microorganisms.