Unlike most municipal wastewater treatment plant (WWTP) applications, industrial wastewater can vary widely from one application to another, even within the same plant, depending on the process being run. Industrial wastewater professionals — especially those facing specific challenges or planned volume increases — can benefit from comparing key points of differentiation in jet aeration system options before making any upgrade.
Challenges, Causes, And Effects
With the goal of aerobic wastewater treatment processes being to nurture microbial activity that reduces biochemical and nitrogenous oxygen demand (BOD) in wastewater, the interaction between those microbes and the oxygen that sustains them is extremely important. Any situation that impacts the balance of that process can limit the effectiveness or cost-efficiency of wastewater treatment.
- Oxygen Transfer Rate. Regardless of aeration methodology, a key measure of effectiveness is how much oxygen exposure is available to the microbial biomass that supports the treatment process. Jet aeration, which uses both low-pressure air and pumped fluid to generate a highly efficient mixture, supports more effective oxygen transfer rates in concentrated industrial wastewater applications than those of bubble diffusers alone.
- Concentrations. It is not unusual for the chemical oxygen demand (COD) concentration of industrial wastewater to range from 1,500 to 3,000 mg/L, and as high as 30,000-50,000 mg/L for particularly demanding food or beverage applications like distilleries. Also, in poultry, beef, and pork processing, the oxygen-demanding nitrogen values are very high. Even if an anaerobic treatment process is first used to reduce mixed liquor suspended solids (MLSS) by 80 percent, that can still leave a heavy load for aerobic processes to break down. In those applications, aerobic treatment systems powered by jet aeration systems are highly effective oxygen transfer systems.
- Troublesome Additives. Surfactants, excess polymer and anti-foam treatments can increase the resistance at the gas/liquid interface of air bubbles and reduce the overall mass transfer coefficient (KLa). Under the turbulent conditions of jet aeration, the liquid film at that interface is continually disrupted and renewed, so that the organic molecules may not become diffusion barriers. In some cases that turbulence results in an enhanced overall mass transfer coefficient. However non-alcohol based antifoam agents are a problem for all submerged aeration devices.
- Energy Costs. While most wastewater treatment professionals appreciate that aeration commands a large proportion of a WWTP energy budget, the options for reducing those energy costs are not always as clear. Jet aeration technology that increases the efficiency of the oxygen transfer rate can support good biological activity at appreciably reduced energy costs.
- Treatment Capacity And Control. Jet aeration can double the treatment capacity of coarse bubble diffusers in a basin of a given size, enabling industries to ramp up production without building new aeration basins or purchasing more air blower capacity. It also offers greater turndown capability for more cost-efficient adaptation to periodic reductions in treatment demands.
How Aeration Affects Performance
The physics of aeration play a large role in oxygenating wastewater biological activity across a variety of conditions — water temperature, basin depth, BOD and nitrogen levels, etc. Bubble size matters to a degree, but conditions within the wastewater and how those bubbles are delivered also impact oxygen transfer rates to the biology.
- Fine Bubbles. The output of fine-bubble diffusers — typically between 1 mm and 3 mm in diameter — offers the highest ratio of surface area to volume for oxygen exchange, and that performance is enhanced by deeper aeration basins. The trade-off, however, is that the oxygen exchange rates of smaller bubbles can be more easily compromised in the presence of surfactants, anti-foaming agents in industrial wastewater applications. Fine-bubble diffusers are also more likely to be affected by a frequent need for maintenance due to clogging or fouling.
- Coarse Bubbles. The larger the diameter of coarse-bubble diffuser output — typically > 6 mm to as high as 50 mm in diameter — the lower the relative oxygen transfer surface area for a given volume of air. Despite this relative inefficiency, coarse-bubble technology is sometimes used as a cost-saving option in less demanding wastewater applications or where the cost of energy is not a big consideration
- Mixing With Fine/Medium-Size Bubbles Enhances Aeration. For maximizing oxygen transfer to get the most production out of the given space in an existing aeration basin, fine/medium-size bubbles (+3 mm) propelled by jet aeration are often the best alternative. By combining pumped liquid and air to mix the contents of a basin system and transfer oxygen through a long exposure path (Figure 1), jet aeration provides a high percentage of oxygen utilization. It is typically enough to double the treatment throughput of an existing wastewater basin while keeping the existing blower and adding a lower horsepower pump.
Figure 1. Jet aeration provides a horizontal discharge stream of liquid and entrained air bubbles to create a longer path and longer exposure time for oxygen exchanges as that stream mixes and slowly ascends through the wastewater basin.
A specialized implementation of jet aeration technology, called slot injector aeration, offers even higher aeration efficiency— typically 15 percent greater — by being able to dissolve the same amount of air with only about 45 percent of the liquid flow that a jet aerator requires. This is particularly attractive for reducing energy requirements in even the most challenging industrial wastewater treatment applications. It also reduces the size, and often the number, of pump stations required.
Alternate Approaches To Common Concerns
Jet aeration systems offer benefits for both new and retrofit installations, thanks to their unique ability to cope with some of aeration’s most troublesome issues.
- Maintenance. In submerged operations, even the best aeration designs will eventually experience maintenance issues related to the harsh operating environment. Look for designs that will minimize those issues as much as possible.
- Fouling. The high velocity of the air/liquid mixture conveyed through jet aeration nozzles and slot injectors discourages scale buildup and fouling, even in high-dissolved solids conditions such as pulp and paper and leachate wastewater treatment.
- Self-Cleaning Features. Jet aeration and Slot Injector™ systems offer the convenience of a back-flush system which clears out accumulated debris by an airlift mechanism, making it easier to maintain high dissolved oxygen levels without having to take a basin out of service.
- Removable Nozzles. With the Slot Injector™ system there is an option for removeable inner and outer injector nozzles for ease in cleaning or replacement after many years of service. This is particularly helpful in applications plagued by severe scale formation and potential clogging due to fine screen bypasses. Leaving the nozzles off the aeration manifolds before onsite installation provides for space-saving efficiency in transit and can greatly reduce shipping costs.
- Chemical Composition. The built-in mixing action of jet aeration helps to minimize resistance at the gas/liquid interface of the air bubbles to maintain good oxygen transfer rates in challenging wastewater applications. This action is particularly important for dealing with many of the chemicals encountered in industrial processes.
- Reduced Capital Expenditures (CAPEX). Jet aeration systems can be wet installed into existing wastewater treatment basins or lagoons and utilized with existing blower equipment to lower the capital cost and basin downtime associated with increased wastewater treatment capacity.
- Minimized Operating Expenditures (OPEX). Jet aeration systems using low-pressure air operate efficiently with fewer or smaller blowers than traditional bubble diffuser aeration systems, keeping energy costs low. Reduced maintenance costs also contribute to lower overall OPEX costs.
Look For Experience
Aside from physical aeration technology, look at the experience of the company and its background in good R&D efforts that result in improvements in materials of construction, reduction in installation, energy and maintenance costs and improvements in aeration/mixing performance. New patents are a good indicator as many aeration companies do not make any investments in product development. Also look for a company with strong industrial references and and when working overseas—good international references — especially references in relevant industry applications. Also look at the longevity of the employees who will work on system development and implementation, and at their familiarity with multiple types of aeration options. Are they experienced enough to observe existing operations and recommend alternatives to solve the problems at hand.
