Air Compressors for Wastewater Treatment Plants

Wastewater treatment plants have two separate compressed air systems. Aeration uses blowers. Nothing to do with screw compressors. Instrument air is the screw machine's turf. Many people new to this industry mix the two up, causing embarrassment during procurement.

Aeration basin depth 13-20 feet. Pushing air to the bottom only needs 6-9 psi. Add piping and diffuser resistance losses, outlet pressure 7-12 psi is enough. Flow rate, on the other hand, is staggeringly large. Municipal wastewater plant processing 13 million gallons per day, aeration volume might need 140,000-210,000 CFM.

Roots blowers dominated this market for decades. Screw blowers in the past twenty years grabbed a good share. Better partial load efficiency. Lower noise too. Magnetic bearing centrifugal blowers increasingly common on large projects. Plants processing over 5 million gallons per day, if electricity rates are high, owners are willing to pay more upfront for energy savings.

Pneumatic valves, pneumatic actuators, positioners, analyzers. These devices need 90-100 psi compressed air.

Instrument air consumption directly correlates with plant automation level. Small integrated equipment might only need 18-30 CFM. Municipal plant processing 5-25 million gallons per day, typically 60-175 CFM. Heavily automated industrial wastewater plants can reach 235-350 CFM.

Pneumatic valve consumption has a characteristic: at steady state, almost zero consumption. Frequent actuation, consumption rises noticeably. Plants with big influent load swings, instrument air consumption considerably higher than stable operation plants. This point easily gets underestimated in selection calculations.

25-100 hp oil-lubricated screw machines cover the vast majority of wastewater plant needs. Municipal plant sweet spot is 40-60 hp, output 70-125 CFM.

VFD or fixed speed? Look at load curve. Plants where instrument air consumption swings big between 40%-100%, VFD machine saves noticeable electricity. Many wastewater plants' instrument air load is actually quite steady. Fixed speed machine with large enough receiver, loading rate can maintain at a high level. Efficiency loss is limited. Whether VFD machine's price premium can be earned back on electricity bills, gotta do the math. Can't generalize.

Oil-free machine occasionally appears in bid documents. Designer writing this requirement probably transferred from pharma or food projects. Brought those cleanliness standards over wholesale. Wastewater plant instrument air using oil-free machine is over-design. Pneumatic valves and actuators aren't that sensitive to trace oil mist. Standard coalescing filter treatment afterward is perfectly usable. Certain analytical instruments genuinely need oil-free air source. Install an activated carbon filter at the endpoint and it's solved. No need to swap the entire system to oil-free for this.

Hydrogen sulfide sources throughout the entire plant. Carried in by sewer network. Released when bar screens agitate. Produced by anaerobic layer in primary clarifiers. Escaping from thickeners and digesters. Released when sludge is being turned in dewatering building.

Compressor room siting should consider prevailing wind direction. Farther from bar screen building, sludge processing building, covered anaerobic structures. Air intake position matters more than room position. Some plants have great compressor room location, but air intake directly faces the sludge storage next door. Equals sucking hydrogen sulfide straight into the system.

Enclosed building with HVAC system, can connect compressor air intake to HVAC supply duct. Air conditioning system itself has filtration. Essentially adds one more line of defense for the compressor.

Compressed air carrying water is universal problem. Water vapor condenses to liquid in receiver and piping, accumulates at low points. Instrument air piping getting water, consequences aren't light: valve internals corrode, positioners malfunction, winter outdoor pipe sections freeze and block.

Refrigerated dryer outlet pressure dew point 37-50°F. Plant where piping runs entirely indoors with heating, this is enough.

Outdoor piping is another story. Aerial pipe rack running a stretch of instrument air pipe. Winter ambient temperature negative 20-30°F. Air with 37°F dew point goes in, water instantly condenses. This situation needs desiccant dryer. Dew point pushed down to -40°F. Regeneration energy is the cost. Heated regeneration type consumes 10-15% of processed volume. Blower regeneration and compression heat regeneration designs can reduce losses. But equipment more complex. Suited for larger volume applications.

Piping endpoint particle filters catch rust flakes, desiccant powder, pipe debris. 1-5 micron precision sufficient for most pneumatic equipment. Oil filtration uses coalescing element, installed before the dryer, prevents oil from contaminating adsorbent bed.

Microbes in biological reactors don't care if the compressor is broken. Instrument air supply interrupted means all plant pneumatic valves lose control. Control valves go to spring return position. Might be full open. Might be full closed. Depends on valve's fail-safe design. Either way, not the position the process needs. Chemical dosing interrupted. Flow out of control. Operators can only run to site for manual operation. Assuming valves have handwheels and they can still be turned.

Pressure monitoring can't just look at the receiver. Pressure transmitters distributed throughout the plant can spot piping issues. Filter differential rising means time to change elements. Pressure drop speed during unload cycle increasing means system has leaks. Record this data continuously. Look at few months' trend, where problems are is obvious.