Industrial Utility Efficiency    

Black & Veatch Specifications from Diffusers to Blowers - Part 1: The Demand Side – Diffusers and DO Control

Blower & Vacuum Best Practices Magazine interviewed Julia Gass, P.E and Patrick Dunlap, P.E. at the Black & Veatch offices in Kansas City during the summer of 2016. Ms. Gass is responsible for aeration blower specifications while Mr. Dunlap is a wastewater process engineer. We held a general discussion covering technologies impacting aeration blowers in wastewater treatment plants, which we have tried to summarize in a two-part article. Part 1 covers DO control strategies and system components like diffusers and valves. Part 2 will be published in the Jan/Feb 2017 Issue and covers aeration blower technologies and the new ASME PTC 13 Standard under development.

 

Good afternoon. Thank you for taking the time to discuss the demand-side of a wastewater treatment plant - as it relates to aeration blower requirements.

Good afternoon. We are pleased to discuss wastewater treatment plants (WWTP) and the technology evolutions we examine to provide our clients with the best possible specifications. Wastewater treatment plants are very significant energy consumers and obviously provide an important service to every community. When new technologies are introduced, there’s always a learning curve for everyone involved; the manufacturer, the engineering firms, the contractors and the wastewater treatment plants themselves.

We do the best we can to stay on top of the new technologies and really cross-examine the claims being made.  We research prior installations to learn about product performance and durability, we inquire as to what kind of service capabilities exist, and we evaluate performance claims. With new technologies, it’s the performance claims which can be the most difficult to ascertain – until the units are shop performance tested.

 

Can you provide some observations on diffuser technology and in particular on diffuser fouling and aging?

Sure.  Diffusers are largely evaluated for their oxygen transfer efficiency (OTE), the pressure drop across the diffuser and their vulnerability to fouling and aging.  Fouling and aging are a facts of life with diffusers and they result in a loss of OTE and an increase in pressure drop over time. To give you an idea of its significance, we use fouling factors which are generally in the range of 0.5 to 0.7 representing efficiency loss over time when specifying diffusers. As part of an aeration design project, process engineers specify the number and type of diffusers in each zone, based upon fouling factors, loading, and a number of other considerations. This design is a big part of what drives the airflow requirements we provide to Julie to specify blowers.

Backing up a bit, most wastewater plants changed in the 1980s and ‘90s to fine bubble diffusers. Most of the original systems had coarse bubble diffusers with larger bubbles of 6 to 10 mm in diameter.  Coarse bubble diffusers are basically stainless steel tubes with perforations. They required more air flow due to a lower OTE but also require less pressure.

Oxygen transfer efficiency improves with decreasing bubble size, which led to the adoption of fine bubble diffusers (producing bubbles less than 5 mm in diameter)starting in the 1980s. The first fine bubble diffusers were made of a porous ceramic material. The big drawback with ceramics is they have a narrower range of air flow per diffuser and, if they are not cleaned and maintained properly,  fouling can be irreversible. Irreversible fouling is more likely if they are required to run at air flows below their low limit.  It’s also more difficult and expensive to clean them compared to membranes.

The membrane fine bubble diffuser technology came into more common use around the year 2000. Most  plants have adopted EPDM rubber membrane diffusers in disc or tube shapes while very few use membranes in other geometries or made of other materials. With membranes excess air can be sent to them to dislodge biofilm (called bumping the membranes) periodically such as once a month to try and clean them.  When membranes foul or age, the fine bubbles become  coarser and we lose oxygen transfer efficiency.  Aging is a unique problem for membranes and refers to the membrane losing its elasticity.  At the end of the day, we have to plan for the worst fouling/aging condition during design to protect the system against fouling and events. 

 

What’s going on with panel diffusers?

EPDM rubber discs are sort of the standard but there is a potential to reach higher efficiencies, whether with diffusers that can get to higher floor coverages or materials which foul & age less severely. One example is panel diffusers which are promoted as the latest-greatest thing due to their ability to achieve lower air flux rates across the membranes resulting in a higher OTE. However they also operate at a higher pressure. Disc diffusers are mounted one foot off the basin floor, while panel diffusers are on the floor and also have a greater pressure drop through the diffuser itself; so you need to consider diffuser type & material in design. An advantage of disc diffuser vendors is their experience and long installation lists with historical performance data from many projects.  There just aren’t that many panel diffuser installations and, therefore, there is less historical data available  on pressure losses in all working conditions.

There’s just a lot going on with diffusers-there are also “mini-panel” and strip diffusers which can also offer greater transfer efficiency through higher floor coverage. Panels have been around for years but the mini-panels are very new and the term is defined differently from one vendor to another. In contrast disc diffusers have lower maximum floor coverages but are less expensive, are durable and people know how they will work.

On a large recent project, we performed a life cycle cost evaluation and silicone tube membrane diffusers were determined to have the lowest life cycle cost. They claim less fouling/aging and are able to install more diffuser area leading to  a higher transfer efficiency and a to lower projected operating costs over a twenty-year life cycle. We have made some creative contractual attempts to hold them to these claims.

 

What are some trends with Dissolved Oxygen (DO) and Most Open Valve Control (MOVC)?

The EPA Fine Pore Aeration Systems Design Manual, published in 1989, first described  these controls. At that time, the most common approach was to control aeration blowers by a header pressure set-point. If blower air pressure doesn’t meet the set-point, blowers ramp up or an additional unit is turned on.  Meanwhile in the basins we have a Dissolved Oxygen (DO) Probe, modulating valve and sometimes a flow meter per zone. It used to be we’d have a complete mix activated sludge system with the intent being to evenly distribute air throughout the whole basin.

On the control side there is room to save energy as well. Depending upon what your permit requirements are, you might be able to turn the air off or reduce the DO requirements, in certain zones or during certain time periods at the WWTP. In the past,  the DO probes would foul and weren’t reliable. The technology has improved and some of our plants are reporting very reliable operation with certain technologies such as the luminescent DO probe.

Some older plants have DO control but not MOVC. When it’s added, you can lower discharge pressure and save, on average, five percent of power consumption. The idea of Most Open Valve-Control (MOVC) is we don’t want to build pressure at the blower just to waste it at the basin valves by  throttling them excessively. The intent is to keep the valve completely open, for the zone needing the most air, and throttle the others just enough to get the right air flow split.

A bigger issue was the MOVC’s were integrated by low bid integrators that may never have worked on a wastewater aeration system before.  While the larger wastewater engineering firms (Black & Veatch and others) are proficient at writing control descriptions, they key to making these system function smoothly is in tuning, time delays, and deadbands.  Setting these parameters requires an integrator with DO control experience. Without that experience,  problems such as valve-hunting frequently occurred. This can happen when a dynamic system changes too rapidly. This results in valves turning on/off constantly and valve actuator motors may even burn out causing reliability and maintenance problems. We are aware of wastewater plants  that simply disabled the MOVC because of the problems[jvg1] .

 

Aeration blower vendors now supply DO controls, don’t they?

Yes and we’d like to think we helped start that trend!  Due to the issues the inexperienced integrators had, we began building DO control integration into the aeration blower specification. Many blower vendors now provide a complete scope including basin control.  Some do it in-house or they work with an experienced integrator so they can provide single-source responsibility. If we know a blower vendor does not have this experience, we will require them to hire one of a short list of integrators as a sub.  We know the integrators we name have the necessary experience.  Specifying  integrators with experience  has resulted in properly operating MOVC systems which save energy and significantly reduce maintenance issues.

 

I’m hearing more about flow-based controls, how’s that going?

A more recent innovation is to control the aeration blowers off of total air flow instead of header pressure. In order to know how much air is going to the basins, we have to sum all the basin flow meters. Blowers are almost never installed with  dedicated flow meters for each. Blowers are programmed to respond to flow requirements as needed and within their performance curves.

The advantages of systems which use total air flow control for the blowers is they are, as a rule of thumb, two percent more energy efficient and provide faster recoveries from a process upset - such as sudden change in plant load. Header pressures vary more with flow-based systems.  With flow-based controls, we have to be very careful with who the integrator is. Do they have experience with  total air flow-based control systems or do they only understand pressure based control?  Do their references indicate their flow-based systems operate reliably?  Advancements made by the blower manufacturers in blower surge controls, including continuous monitoring by PLCS, are also making flow-control more possible without the risk of forcing a blower to surge.   

 

What impact are Biological Nutrient Removal processes having?

Before Biological Nutrient Removal (BNR) processes were common, aeration basins tended to be more like completely mixed basins and over-aeration wasn’t a process detriment. In these basins a single point of aeration control was considered adequate if energy was not a big driver; for example when power was cheap. This strategy isn’t good enough for BNR processes where over-aeration is not only costly but also detrimental to the process itself. If we are doing a BNR process, we have a recycle from the end of the aerated part of the basin back to the unaerated part where the nitrogen is removed and having oxygen carrying over into anaerobic or anoxic basin zones results in diminished performance. You have to get rid of oxygen to do this and independent control of DO levels throughout the basin becomes important. We have to taper the air flow for the series of zones we now have in each basin and get more air at the upstream oxic or aerated zones because it is a plug flow process.

Now that we aren’t distributing air equally to each basin, we do need a modulating valve and flow meter per zone. The purpose of the flow meter is to provide feedback to the system to which indicates whether the DO/air flow set point is being met. We have two control loops- one per zone in the basins plus the blower control loop based on either header pressure or total air flow. You want these two control loops to interact. If the zone with the most-open valve is getting too much air, in a pressure-based control system, the header pressure needs to adjust downwards slightly (like 1/10th of a psi), so blowers deliver less air.  You want the two control systems to interact, and they interact at the blower header set-point.

 

How do you set the DO specification for BNR?

The process must provide adequate oxygen to fully nitrify (fully convert ammonia to nitrate).When flows and ammonia loads increase, the process must convert more ammonia to nitrate in a shorter retention time. This will require more air but can also be ensured by reaching the DO concentration adequate to ensure the maximum biological rate.

However, there are advantages to cost and performance to operating at lower DO set points.  If you want to realize the benefits of lower DO operation but need to nitrify consistently, then you need to have dynamic controls to respond to changes in ammonia loadings.  This might mean the plant can decrease the DO set-point from the standard of 2 mg/L to 1 mg/L or even below, for example.  A rule of thumb on energy savings we use is this;  a 1 mg per liter drop can translate to a 12% reduction in power consumption. So if you reduce the DO setpoint to 1 mg/L, you can reduce energy consumption by 12%.

 

Do most plants have a fixed DO level?

In the early years, most plants had a varying DO because it simply wasn’t controlled! Now we are varying DO levels from zone to zone. If there’s a wet weather or loading event, an ammonia reading can drive a higher DO set-point. Zone 1 will normally have a higher DO requirements, while in the later zones we might be able to run lower DO levels and save some energy. For example, the DO Level might be 2 in Zone 1 while it’s 1 in Zone 2.

Other factors impacting DO level requirements to fully nitrify are inhibitory constituents in the wastewater and the regulatory permits driving local effluent requirements. Most of the things we are talking about aren’t going to reduce blower sizes and maximum air flow requirements as we still have to account for diffuser fouling/aging and maximum loading conditions/events. What they do impact, however, are average DO level and aeration blower power consumption.

 

This sounds like ammonia-based control.

Exactly; and there are some new ammonia based control systems/concepts coming out. The idea is to analyze ammonia somewhere in the process and utilize the ammonia information to adjust DO. You can then program the controls and respond to changing conditions  using either a feed-forward or feed-back loop on ammonia. Some companies specializing in wastewater process controls, as well as the larger blower vendors and their controls subs are developing software to do this. The reference list, however, is short. As with all new technologies, evaluation is important to determine if it is right for you. Maintaining ammonia analyzers is a time-consuming task which must be done frequently.  In some cases, the plants do not have the maintenance staff with time to handle this.  However, as energy costs continue to increase and plant discharge permits become more restrictive, a life cycle cost analysis may be warranted to determine whether the additional maintenance is cost effective.

 

To learn more about Black & Veatch visit www.bv.com.

 To read similar articles on the Wastewater Treatment Industry visit http://www.blowervacuumbestpractices.com/industries/wastewater.