Getting the right air ration

Subscription required

As a guest you can read up to 3 full articles before a subscription is required.

You can read a further 2 articles for free.

Subscribe Now, Sign up for a Free Trial, Log In

The wastewater aeration market is undergoing big changes. Gordon Cope looks at the challenges and opportunities.

Wastewater treatment for the municipal and industrial sectors is one of the largest portions of the water market. According to GWI, global capital expenditure is in the range of $25-35 billion annually; of that, aeration equipment represents about 10%. “Aeration is the backbone of wastewater treatment,” says George Smith, director of biological services for Siemens Water Technologies. “It is the most efficient way of treating biological matter. That is why you see it in over 90% of treatment plants in North America.”

 
But aeration, like most other valuable tools, is facing challenges – not least from rising energy costs. “Wastewater plants use a lot of energy, and 70-75% of that energy is consumed by the aeration process,” says Smith.
 
Fortunately, much can be done. Manufacturers are creating more efficient kit and new techniques to optimise existing technologies. “You don’t necessarily need to install new equipment to make gains,” says Smith. “Huge gains can be made through the way you manipulate your process.”
 
Down the drain
Every day, municipalities, factories, beverage plants and other facilities produce billions of litres of wastewater. It can contain a wide range of solid material, faecal matter, dissolved ions, metals, organic and nonorganic compounds. Municipal wastewater treatment forms the bulk of aeration use. Most treatment processes involve filtering, cleansing and disinfection.
 
The first stage, known as pre-treatment, uses screens to remove plastic bags, cans, sticks and other solid objects that collect in the sewer system. The next step is primary sedimentation.
 
Untreated wastewater enters large tanks where sludge settles to the bottom (where it is scraped away and pumped to sludge digestion facilities), and grease and oils float to the top, where they are skimmed off. Primary sedimentation gen erally removes up to 70% of suspended solids.
 
The wastewater, or liquor, still contains significant amounts of soap, detergent and other organic material (generally referred to as biological oxygen demand, or BOD). If released into surface waters, the BOD-laden liquor would promote the unregulated growth of bacteria and rob the water of oxygen, leading to aquatic kill-off.
 
It is therefore subjected to secondary treatment in closed basins (lagoons) and cement tanks, where bacteria and oxygen are mixed with the liquor under controlled conditions in order to degrade the BOD into inert compounds that can be removed from suspension through gravity. Over the course of several days, the BOD is reduced by over 90%. Excess quantities of nitrogen and phosphorus are also removed, and the water is treated with disinfectant. At this point, it can be safely discharged to surface waters.
 
It takes about 1.5 kg of oxygen to treat 1 kg of BOD, and naturally dissolved oxygen is soon depleted by bacterial action. Treatment plants therefore incorporate some form of aeration to increase the amount of oxygen available. The most common systems are surface aeration and diffusion aeration. Surface aeration, or SA, sprays water into the air in order for oxygen to dissolve into the water.
 
Major manufacturers of SA systems include Siemens and Aqua-Aerobic Systems. “The Siemens Aqua-Lator is installed in thousands of systems throughout the world,” says Terry Johnson, sales manager for Siemens. “Surface aeration is simple, relatively inexpensive to purchase, quick to ship and install, and doesn’t require advanced technical training to operate. It is flexible, and a proven workhorse.” SA is often used in smaller rural plants (50,000-500,000 GPD (190- 1,900m3/d)), where land scarcity is not a consideration, and higher operating costs per treated gallon are offset by lower upfront capital expenditures.
 
Diffusion systems are typically found in medium (1-50MGD; 3,785-190,000m3/ d) and large (100-500MGD; 378,500-1.9 million m3/d) plants. These are classified either as fine bubble or coarse bubble systems. These are mounted onto air manifolds or pipes running near the bottom of the tank, either on one side of the tank, or sometimes in a grid system to provide more uniform aeration.
 
“Fine bubble diffusers are more expensive and higher in maintenance, but their greater surface area allows for higher amounts of oxygen dissolving into the water, which increases the efficiency of the system by a ratio of 2:1,” says Smith.
 
Surface and diffusion aeration have limitations, however. The SA process sprays water into the air, and if the air is too cold, then it cools down the water to the point where the biological process stops. Diffusion systems are therefore used in colder climates, but are more expensive. Both use large amounts of energy, accounting for more than half of a wastewater treatment plant’s power consumption.
 
SA also requires relatively large amounts of land. Over the last decade, in an effort to handle increasing loads without expanding the physical area, many wastewater treatment systems have been adding various technologies to the secondary treatment step, including membrane bioreactors (MBRs), which separate clean effluent from the liquor by using a vacuum to draw the water through fibre filters.
 
GE Water & Process Technologies, Siemens and Kubota are major suppliers of MBR systems. GE has approximately 1,000 Zeeweed MBR technology systems in operation around the world.
 
“Conventional systems have two main functions,” says Jeff Peeters, a senior product manager for GE. “The first is the biological process, where oxygen is diffused into the mixed liquor to enable the metabolism of the bacteria that degrade the organics in the wastewater. This is typically done through fine bubble aeration.
 
“The second component is separation of the treated water from the bacteria and other solids through gravity in a large tank, called a secondary clarifier. An MBR system differs from a conventional system in that it performs the solids-liquid separation using ultrafiltration.”
 
MBR has its limitations, however. The technology requires larger upfront capital costs and operating costs (MBR systems consume large amounts of electricity and require considerable maintenance), which frequently makes them unsuitable for developing countries, where wastewater treatment competes for funding with other infrastructure investments.
 
Energy savings
Much is being done to address aeration’s energy consumption. Mapal Green Energy, based in Israel, makes an advanced form of aerator called the CNM floating fine bubble system.
 
“A fine bubble system creates hundreds of millions of small bubbles that mix deeply in the reactor,” says Zeev Fisher, vice president with Mapal Green Energy.
 
“You achieve aerobic action in the whole volume of the reactor, instead of just the surface. You have six metres of action with bubbles (the depth of a large secondary treatment tank), versus one to two metres with SA. This creates a much higher energy efficiency. You deliver 0.9 - 2.1 kg of oxygen per kWh with SA, and 3.6 - 4.8 kg with fine bubbles. That’s four to five times more efficient.”
 
Mapal has more than two dozen installations in Israel and Africa. One system was recently installed in Ramat Hasharon, an Israeli city with 50,000 inhabitants. “They have a system that treats 10,000m3/d in two biological reactors,” says Fisher.
 
“It uses about 110 kWh in energy per reactor. We installed two blowers with eight floating fine bubble aeration units while the system was in operation. The system is easy to operate, and has sensors that automatically adjust the air flow rate as per the dissolved oxygen level in the water. It now uses 50-60 kWh. It’s also possible to increase the wastewater flow rate by almost 50%, since it is a modular system and it is very easy to add more units. The plant, which is privately owned, pays us back through a percentage of the saved energy costs. The return on investment is around three years.”
 
Sorubin, based in Sweden, has also pursued energy efficiency and simplicity of design in aeration systems. The company’s recently launched Microluft aerator – which already has a clutch of references in Sweden – incorporates a bottom-mounted impeller that creates a vortex that mixes air and water, creating a foam that promotes BOD reduction. Sorubin’s aerator claims to reduce the amount of energy consumed by more than 80% in comparison to conventional aerators. “Our system delivers more oxygen with less energy,” says Stefan Sandström, CEO of Sorubin. “That’s a tremendous saving of operating costs when you consider that in some jurisdictions, such as Indonesia, electricity can cost as much as $1.00/kWh.”
 
The Microluft module, which costs in the $20,000-25,000 range, and can deliver up to 3-5 kg of O2/kWh, is marketed primarily at the industrial leachate treatment sector. Sorubin is now turning its attention to the municipal market. “The aeration step requires a lot of energy, so it’s the most expensive process step in any wastewater treatment setting,” says Sandström.
 
“In Sweden, the required energy expenditure to achieve enough aeration is 22 kWh per person per year. This means that a city of one million citizens will spend 22,000 MWh per year on aeration treatment for their urban wastewater, costing in the vicinity of €2 million per year. On average, Sorubin’s aeration technology can reduce energy expenditure by 55%, leading to savings on electricity of over €1 million per year. This means that energy expenditure will be lower, leading to a smaller carbon footprint, and the possibility to fund other projects.”
 
MBR manufacturers are also addressing high energy consumption. “You need to employ air scour to keep the membranes clean,” says Peeters. “In the past, this aspect of the MBR process used a lot of energy. Through R&D, we have designed the system to reduce the amount of air scour necessary to maintain efficiency. This has reduced energy consumption by 30% compared to our previous generation MBR technology.”
 
Earlier this year, GE launched the LEAPmbr system, which combines aeration and separation functions. The basic building block is a membrane module that connects thousands of reinforced hollow-fibre strands to a header and footer collection assembly.
 
The modules are then amassed in cassettes, each of which can hold up to 48 modules. The cassettes can then be installed in process trains to handle large volumes of throughput.
 
The membranes are made of reinforced hollow fibres that, under vacuum, allow water to pass through, but reject particulate material greater than the pore size of the membrane (0.04 µm). Air scour is used to move concentrated wastewater away from the membranes in order to maintain efficiency.
 
The new design requires one third of the plant footprint compared to a conventional treatment system, says Yuvbir Singh, general manager of products and systems for GE Water. “It is also a simplified design that reduces the amount of aeration equipment and controls by 50%. It also reduces energy consumption by 30%. In the end, you have better quality water, a smaller footprint, less energy use and lower operating costs.”
 
New processes
Not all solutions require exotic new hardware. For the last several years, Siemens has been promoting a hybrid aeration system that leverages the surfactant action of fats and soaps in the waste stream. In a diffused aeration system, surfactants tend to coat the air bubble and prevent oxygen from dissolving easily into the water.
 
This means that, at the front end of the plant where fats and soaps are found in greater quantity, it takes a lot more air from a diffused aeration system to do the same job.
 
“On the other hand, surfactants tend to make sprayed water form smaller droplets, which increases their surface area,” says Smith.
 
“If you put an SA at the front end where surfactants are more concentrated, it is more effective in dissolving oxygen. As the biological action breaks soaps and fats down, their concentration decreases, and fine bubbles become more effective.
 
“A hybrid system simply starts treatment with an SA then finishes with fine bubbles. By leveraging the surfactant effect, you can save 30-40% on energy costs. We have installed a major hybrid system in Turkey.”
 
Siemens has also been promoting a process called aerated-anoxic. “Most plants keep a high level of dissolved oxygen (DO) throughout the aeration process,” says Smith. “But it takes extra energy to maintain that level. You can, however, run tanks in series, where the first tanks have just enough aeration to maintain zero dissolved oxygen – everything gets used up by the biological process. Later tanks have a high dissolved oxygen where the demand is lower. You can save 30 - 50% energy using the aerated-anoxic process.”
 
According to Siemens, installing an aerated-anoxic process can achieve significant gains within existing plant infrastructure. “A plant in Wisconsin with a fine bubble aeration process reduced their delivery and maintained zero DO for the first two thirds of their process, and only a positive DO near the end. Eff luent performance was just as good, if not better, with an energy saving of 40%.”
 
Aerated-anoxic can also be used to reduce nitrates in water, which cause algal blooms and harm aquatic systems. “Most nitrogen entering a wastewater treatment plant is in the form of organic nitrogen and ammonia,” says Smith. “The aeration turns it into nitrate, which then acts as an algae nutrient. In order to denitrify, you need a low oxygen level, which causes the biological process to strip oxygen from the nitrate, leaving harmless nitrogen gas to disperse into the atmosphere.”
 
State legislation is pushing many plants toward lower nitrate levels. “A plant in Jackson, Mississippi, was looking at adding tanks and a large investment to reduce its nitrates, but we have a proposal in to them where we could reduce nitrates using their existing tanks,” says Smith. “This would involve separating out aerated and anoxic processes in series, instead of running the tanks in parallel. The nitrate levels will fall, and the 30% energy savings is icing on the cake.”
 
Although significant progress has been made to reduce energy, facility size and capital costs, many aeration innovations face industry resistance.
 
“Our main obstacles are the consulting companies that base their fees on capex,” says Fisher. “A typical example is a lagoon in the US that is causing a bad odour. It would cost us $2 million to fix the problem, but the consultant recommends a new $30 million plant because it helps his bank account.”
 
“Many plants are upgrading equipment to save energy and increase the quality of eff luent, so we are seeing first adopter utilities experimenting with aeratedanoxic and hybrid aeration,” says Smith. “It is, however, a very conservative sector, and many operators want to see someone else do it first – they don’t have a high comfort level.”
 
Future opportunities
Aeration companies looking for growth opportunities are increasingly turning to developing countries. GE envisions an increasing portion of MBR business being conducted in China, India and other emerging geographies. “These areas want top-of-the-line technology, and to see that there are operational savings to be had,” says Singh.
 
Economical systems will also make headway. “We are following up lots of leads in Africa, South America and Asia,” says Fisher.
 
“For example, in Mumbai, one of the lagoon WWTPs handles 90,000m3/d, primarily through in-and-out lagoons. It uses 1,400 kWh. They want to upgrade to 250,000m3/d, but they are flanked by a nature reserve, so expansion space is limited. We put together a proposal where we reconfigure their system to handle 250,000m3/d, but only using 1,200 kWh. We avoid the cost of a new plant, which would be $250-300 million, versus the cost of retrofitting and upgrading the plant based on our system – about $50-60 million. The system is robust and easy to maintain. We use the same space, and we save a lot in operational costs. And we do a live upgrade, while the plant is in full operation.”
 
Despite the innovations, however, SA is likely to remain a major force, not only in mature markets with large installed bases such as Europe and North America, but increasingly in developing markets, towards which Siemens foresees a marked shift in new-build activity over the next 10-20 years.
 
“Many developing countries don’t have huge amounts of money to purchase expensive equipment, but they do have lots of land, and a large need for wastewater treatment,” says Johnson. “Surface aeration is low cost, and the aeration basin is typically earthen with a plastic liner. This allows developing countries to build hospitals, schools and roads, but still have a good wastewater system.”
 
Further down the line, manufacturers hope to revolutionise energy usage. “We are working on processes where we may someday turn wastewater treatment plants from energy hogs into energy sources,” says Smith. “This involves a host of factors, from diverting sludge prior to the aeration process into anaerobic digesters, which produces gas and reduces aeration, to using aerated-anoxic and other processes to reduce energy consumption.
 
“I was in Hong Kong recently. They only have primary treatment, and pump the rest out into the bay. Now, they need to add secondary treatment. Land is very expensive, and a conventional process would require a very large plant to handle the load. We worked out a system on paper that would allow them to build secondary treatment and a digester within the limited area available, and end up delivering a surplus of energy. Instead of spending $20 million per year on operating costs, they would end up with a $5 million recovery.”
 
Participants in the sector are confident that aeration will continue to be a foundation of wastewater treatment for a long time.
 
“City water managers in arid regions are realising it is more cost-effective to treat wastewater to a higher level and reuse it, rather than simply treat and discharge it, due to regulation,” says Singh.
 
“For a small incremental cost with an MBR system, they can repurpose wastewater to constructive reuse.”