Aeration system is safer but also cost-effective

Hydro Innovations

Wednesday, 01 April, 2020



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Designers have many options to choose from when designing an aeration system. They can select surface aerators, which could be slow-speed or fast-speed units. They can also select surface aerators with draft tubes, submersible aerators, diffused air systems or a Venturi Aerator.

All systems have their positive points and all have negative points. If designers opt for a diffused air system, which is a positive choice in deep tanks or basins, they are very efficient, but one of the most expensive systems. Surface aerators are a cheaper option, but many have drawbacks with efficiency, life cycle longevity and operator safety.

The Venturi Aerator can deliver dissolved oxygen (DO) and mixing capabilities on par with or exceeding those of surface aerators, but offers increased safety levels for operators and longer service life of the equipment employed.

Efficiency

For efficiency, it is hard to beat a diffused air system, but they do have their negatives. The blower used is generally noisy, requiring its own building, and the system has a high capital cost. These systems are also expensive to service.

A Venturi Aerator from Venturi Aeration can produce up to and exceeding 1.86 kg O2 per kWh, depending on the pump selected and whether the pump is on a suction lift. Hydro Innovations, distributor of the Venturi Aerators, uses Gorman-Rupp self-priming centrifugal pumps which can produce hydraulic efficiencies up to 88%. This compares very favourably to surface aerators and also delivers a safer environment for operators and reduces maintenance costs.

Safety

A Venturi Aeration system is located on the banks of lagoons or outside tanks, not in them or on them. Operators can therefore access equipment for maintenance without the need for cranes (bringing power lines etc into play), boats or floating walkways. There is never a need for the operators to be exposed to falls into wastewater treatment tanks, falls from ladders or falls from failed safety infrastructure. And because the pumps powering the Venturi Aerators are Gorman-Rupp, they have been built with operator safety in mind. This includes pressure relief valves and clearance adjustments without the need for even opening the pumps. This improved safety also comes with a decrease in maintenance costs.

Also, the splasher-type aerators splash/spray water into the air as a function of how they operate. Windy days could cause aerosol-size droplets to be carried by the wind and transmit bacteria to unwanted places. A Venturi Aerator on the other hand can ‘inject’ oxygen-saturated water below the surface to eliminate these risks.

Large splashing aerators are tethered and difficult to safely access.

Maintenance costs

Maintaining a surface aerator can be costly. Any time they need to be accessed, a crane or boat is required to reach them. Smaller, lighter floating units are sometimes tethered so that they can be winched to banks, but this can be time-consuming and not without risk. Some surface aerators are driven by gearboxes, which require routine maintenance to avoid premature failure. Diffused aeration systems require tanks to be drained for the periodic replacement of membranes, a lengthy and costly exercise.

Venturi Aerators have no moving parts, so only the pump requires maintenance. And because pumps are easily accessed, clearance adjustments only take minutes and oil changes are simple, maintenance costs for these systems are much lower than any other aeration system. Another aspect of cost is the amount of time the equipment is expected to last.

Equipment service life

Venturi Aeration systems can be expected to deliver mixing and DO transfer for up to, and in many cases exceeding, 25 years because of the robustness of the construction and the ease of access to allow operators to keep the equipment in peak condition. It is not uncommon for a Gorman-Rupp pump to still be in operation 25–30 years after installation. This is not the case with surface or floating aerators or submersible units, where the difficulty of access inhibits routine maintenance and shortens equipment life. Even the support structures for surface aerators or submersible aerators can be affected by the corrosive environment they work in, and need replacement well before asset owners would expect them to.

Capital costs

A Venturi Aerator and Pump are not inexpensive when compared to some surface aeration systems, but the infrastructure and installation costs are generally much lower. All that is required is a concrete plinth for the pump to be affixed to, and some piping and supports. Installing a surface aerator will require overhead cranes and possibly floating or permanent access platforms. A submersible system will require a separate sump, guide rails and lifting chains. A diffused air system is expensive and the infrastructure and installation costs are also high.

A diffused air system is capital and labour intensive. A platform is constructed to access splasher aerators.

Versatility

The Venturi Aeration system is the most flexible of all aeration systems. It can be used for mixing and aeration and, after process (particularly in batch systems), can be used to pump the treated water to the next stage, just by switching a valve. The Venturi Aeration unit can either be driven by a dedicated pump or a very large (and more efficient) pump can drive two, three or more aerators. This enables the power unit (pump) to be in one location while aerating at several points that could be hundreds of metres apart. This has the potential of saving on power cable runs and other infrastructure.

Valves can direct flow to a Venturi Aerator or discharge it.

If the aeration system is temporary, a Venturi Aerator and Gorman-Rupp pump can be used for that purpose, then easily repurposed as a pump after the aeration project has been completed. The Venturi Aeration system is also excellent at destratification, as the suction and discharge points can be set wherever the operator wants them.

The Venturi Aerator can also be used as a vehicle for transferring pure oxygen into bodies of water. By using oxygen generators or liquid oxygen tankers with vapourisers, Venturi Aerators can transport up to four times their usual volumes. This could be ideal for systems affected by seasonal conditions (as in the wine production sector), where units can be used in their usual way during the low periods and then ‘supercharged’.

A Venturi Aerator is temporarily ‘fed’ pure oxygen.

In conclusion

The Venturi Aerator is an efficient and cost-effective form of aeration. It is also the safest for operators to monitor and maintain, and is easy to install. The system is also easy to scale up and to greatly increase oxygen transfer, and is extremely versatile in its application and use.

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