Wastewater recovery key to high resistance
Ian Crossley, Chief Engineer of Zenon, Ontario, Canada has been involved with water treatment plant design and construction for over three decades. He looks back on his career and the rise in the use of membrane technology.
Over the years, our main sources of drinking water - rivers, lakes and reservoirs - have been encroached by urban development. The runoff from farms, leakage from house septic systems, and other contamination has led to a decline in water quality. Additionally, it was discovered that pathogens, such as Giardia and Cryptosporidium, are difficult to remove by traditional treatment processes and extremely resistant to normal disinfection practices. Regulations introduced over the last two decades established high treatment standards, specifying that surface waters should be clarified, filtered and disinfected as a minimum.
In North America, small towns and villages were affected by these rules and regulations, and had to stop using reservoirs if they could not afford to build new treatment facilities. This put a greater burden on groundwater supplies, often depleting local aquifers. Funds were made available by the state and federal authorities to help small communities to cope with these changes.
MF Comes of Age
Thus market and legislative factors came together and provided the driving force for microfiltration (MF) system sales to take off in the USA. The standardized and modular MF system designs allowed small communities to meet treatment requirements economically and reliably.
All these MF systems use pressurized containment vessels for the membranes and some form of backwash, either with air or water or both. Most systems operate at an overall recovery of about 92%, that is, 8% of the incoming water is wasted or is further treated to enable it to be recycled.
In the background, ZENON Environmental was working on a very different approach - an exposed hollow fiber about 2 mm in diameter that is operated under vacuum. Oddly enough, the roots of this membrane development were in wastewater recovery. Consequently, the membrane systems developed by Zenon are very resistant to solids and difficult to foul.
The membrane fibers are loose and are immersed vertically in a tank of water, while air is injected beneath to provide turbulence in the fiber bundle. Externally applied coarse bubble aeration maintains the flow through the membrane and helps prevent fouling.
Vacuum is applied on the inside of the fiber, drawing filtered water through the membrane from the outside to the inside (outside-in). The filtrate is collected in headers and passed to the clearwell or treated water reservoir after disinfection. Concentrated wastewater is drawn off continuously either by gravity or by pumping. Overall recovery is in the region of 95%, but can exceed 99% with extra treatment.
Every 30 minutes or so, the filtrate flow is reversed and a backpulse is applied to the inside of the fiber. This forces deposits off the outside of the membrane, and returns the membrane to a lower operating differential pressure. Eventually, the membrane builds up deposits that cannot be removed by a backpulse and a chemical clean is carried out to return the membrane to its normal state. These 'recovery' cleans take place approximately every 30 days of continuous usage. A key feature of the Zenon membrane is its resistance to high chlorine levels. The reinforcement built into the fiber structure makes it very tough and hard to break.
At first, the benefits of immersed membranes were not economically apparent. However, as water applications became larger and larger, the modular pressurized systems suffered the fate of most desalination membrane systems - they are not subject to 'economies of scale'. With most membrane systems, as the plant capacity increases, the capital cost of the equipment follows almost linearly.
Economies of scale
Immersed membrane plants do not suffer from these economic 'rules'. They do achieve economies of scale. Fewer, larger valves and pipes can be used. Permeate pumps can be larger, concrete tanks and channels can be used, and so on. Immersed membrane systems are more economically suited to large water treatment plants than pressurized systems.
The other 'rule' that is broken is that membranes cannot be used with coagulants or high solids concentrations. The use of an "outside-in" fiber, mounted loose in the water, and air agitation, allows much higher solids loadings; in wastewater these can be high as 20,000 mg/l, although for more optimum performance, usually 10,000 mg/l solids is used.
This has enabled coagulants like aluminum sulfate and ferric chloride to be used to remove colloidal organic matter and turbidity ahead of the membranes. This means that membrane treatment is no longer a unit process that is provided in addition to traditional processes, but is now able to replace them.
Why are more and more municipalities and large industrial users of water buying
membrane plants instead of traditional processes?
The reasons may be summarized as follows:
- Lifecycle costs are similar
- Footprint is up to five times
- Easily expanded or retrofitted
- Water quality is superior
- Operationally simpler
- Robust performance
If site space is constrained or land is expensive to purchase, then immersed membrane systems will tend to be a preferred alternative. Often existing rectangular tanks can be modified and used instead of building new. The modularity of the systems allows easy expansion provision. The water quality resulting from a membrane plant will be more consistent (almost irrespective of raw water quality), and the physical barrier of the membrane provides assurance that pathogens, such as Cryptosporidium, do not pass through. Operators like the reliable fully automatic operation of the faculties. The process is robust and can withstand short-term excursions without serious consequences, and is inherently reliable.
ZENON Environmental has recently been selected by the South San Joaquin Irrigation District, CA to provide an engineering design contract for a 36-mgd (135 Ml/d) water treatment plant, and has a 25-mgd (95 Ml/d) drinking water plant in service in Olivenhain, California.
Membrane treatment for water and wastewater is here to stay and growing fast. The key drivers are economic, water quality improvements and space saving attributes of membrane systems. Large scale plants are in service and under construction. The proprietary nature of the technology does not seem to be holding back its market growth.
Traditional processes do not have the same inherent benefits that membrane treatment offers. Time will show that 21st Century processes such as immersed membranes will replace the clarifiers and rapid gravity filters of the 20th Cenury.