SELECTING THE MOST EFFECTIVE BLOWER FOR WASTEWATER APPLICATIONS
Challenges of a Waste Water Treatment Plant
In a typical wastewater treatment plant, approximately 60% of the total energy demand is used for aeration. Hence, it is essential for every wastewater treatment plant to invest in the appropriate blower to maximise effectiveness and minimise energy expenditures.
An accurate cost-benefit analysis must include the capital expenditure of the aeration blowers as well as the operating variables. Consider daily and seasonal swings in oxygen demand, fouling and aging of diffusers, air flow control and turndown capabilities, total blower efficiency and energy consumption over time, mode of operation, blower accessories, and plant set up when making your decision.
Operating variables can significantly affect cost and benefit. With energy efficiency as the primary driver of aeration blower technology, the goal of this paper is to illustrate the most efficient and cost-effective way to achieve energy efficiency based on real-world applications and right-sizing blower technologies.
An Overview of Blower Technologies
There are four main blower solutions for wastewater aeration applications:
• positive displacement blowers;
• turbo blowers;
• hybrid blowers and;
• a combination of blower technologies.
POSITIVE DISPLACEMENT BLOWERS (PD Blower)
Often referred to as “the workhorse” for its flexibility to perform well despite changing conditions, the PD blower has a lower
initial cost than its turbo and hybrid counterparts. However, the PD blower‘s lower cost can be offset by higher energy consumption.
Aerzen Turbo at 200 - S10
Most efficient when operating near its design point and therefore performs cost-effectively in applications with narrow swings in turndown. The turbo, however, has a higher initial cost than its rotary lobe and hybrid counterparts. For applications with lesser variations in operating conditions, the additional cost can be compensated for by lower energy consumption.
HYBRID BLOWERS (ROTARY LOBE COMPRESSORS)
Provides energy efficiency comparable to turbo with the flexibility of PD technology. The hybrid uses a low-pressure screw rotor instead of a rotary lobe rotor. In applications with large swings in flow and pressure, hybrid blowers can achieve rates as low as 25 percent of the original design point.
COMBINATION OF BLOWER TECHNOLOGIES
For large WWTPs, applying a combination of blower technologies at various points in the treatment process can improve overall energy efficiency and reduce initial and long-term operating costs. This solution can be especially effective in a retrofit application and in processes where the duty cycle on the blower can be as low as running only one hour per day.¬
• Turbo blowers are ideal for applications where they can run at the same speed all day — they are less efficient when used in applications with regular fluctuations. The best way to optimise a system is to combine a turbo (for base load) with a hybrid (for peak load and low flow conditions).
• It is critical that the turbo is able to tolerate the introduction of the hybrid without surging. The idling feature and the current-based inverter control easily facilitate this combination.
• It is also critical that the PD or hybrid be equipped with pulsation attenuation, to minimise the disturbances in the system while operating with the turbo.
• Overall energy efficiency can be higher with this approach, and the overall turndown range can be extended to 6:1 or more.
WWTP Application Example: Bremervörde Sewage Treatment Plant
Combination Blower Technology using Aerzen Turbo Blower and Aerzen PD Blower
Overview: The Bremervörde sewage treatment plant in Germany has an overall design capacity of 30,000 Einwohnergleichwert (EGW, or population equivalents), which is a measurement of the total pollution load divided by the individual pollution load of one person. Operating at approximately 29,000 EGW, the plant is nearing full capacity, processing up to 3,000 cubic metres of wastewater per day. However, weekend turndown can result in load fluctuation from 1,200 to 1,500 cubic metres. During the week, the plant processes a nearly constant 1,500 cubic metres per day using two existing Aerzen Delta PD blowers. Approximately 75 percent of the plant‘s energy consumption goes toward generating process air, which represented a significant operating cost reduction opportunity.
Objective: Optimise energy use by implementing a fully automated blower system that would meet process air requirements within the operating range of 50 to 100 percent.
Results: The plant selected the Aerzen AT 100 turbo blower. The new turbo blower serves as a base load generator for process air, operating at a capacity range from 35 to 80 cubic metres per minute (1200 to 2800 SCFM). The two existing PD blowers are connected to the new system and automatically start when needed to handle peak loads or serve as redundant blowers. Adding the new turbo blower resulted in cost savings on the order of 20% to 25%.