Example installment of Aerzen positive displacement (PD) blowers with package mounted variable frequency drives (courtesy of Aerzen).
A small site located within a floodplain, prone to erosion, and currently occupied by an existing in-service wastewater treatment facility is not at the top of any engineer’s list for a desirable site to expand a wastewater treatment plant or reclamation facility. However, these challenges created opportunity for specialized solutions during the design of the facility expansion; in particular, in designing the aeration and digester blower system.
A regional Wastewater Treatment Plant (WWTP) located in northeast Houston, TX was reaching treatment capacity due to ongoing growth in the municipalities and increase of organic strength. The existing WWTP is permitted for 0.95 million gallons per day (MGD), after factoring the current wastewater demand projection, the facility ultimately needs to expand to 2.25 MGD. The WWTP expansion was split into two phases with a modular design to facilitate an incremental expansion paced by growing demand. The ultimate expansion of the WWTP had to account for multiple site constraints including buffer zone, floodplain and floodway regulations, future nutrient removal facilities, as well as an on-site detention and mitigation pond. Operator and client preferences led to the selection of a conventional complete mix activated sludge treatment process with single phase nitrification.
Two-Phase Expansion to 1.875 MGD
Phase I expansion will increase the overall treatment capacity of the facility to 1.875 MGD. New construction includes three (3) aeration basins, three (3) clarifiers, and two (2) chlorine contract basins as well as an elevated building to house all blowers and the motor control center (MCC). The existing 0.95 MGD treatment module will be reconfigured to become a multi-stage aerobic digester upon completion and start-up of the proposed new treatment modules.
Phase II includes the construction of one additional treatment train (aeration basin, clarifier, chlorine contact basin) to increase treatment capacity to the ultimate flow of 2.25 MGD. Also factored into the ultimate site plan design were future anaerobic and anoxic biological nutrient removal (BNR) basins and tertiary filters in anticipation of stricter nutrient removal requirements being enforced in the future.
The overall expansion project also had goals of alleviating existing flood risks, automating operational components, and improving energy efficiency. Improving the aeration system’s efficiency was the main method for achieving energy efficiency improvements. The proposed blower system design needed to supply all the process air needed for biological treatment and various mixing applications while accommodating future phases of expansion.
Engineering Around Site Constraints
The existing multi-stage centrifugal blowers and the motor control center were flooded during Tropical Storm Harvey, inundated by over four feet of water, and were repaired and put back into service without any mitigation measures being put into place. To minimize future flooding risk, a new elevated building to house the electrical equipment and centralized blower system was designed.
There was little space left over for new Blower/MCC building after accounting for the various site constraints. This limitation was addressed by locating all the aeration and digester blowers in the same room and installing the variable frequency drives (VFDs) directly on the blower packages thereby reducing the overall footprint of the building. The combination of these items into one central location also offers several operational and economic benefits including a reduction in the length of shielded instrumentation cables and shorter runs of electrical cables and conduit between the between equipment and the control panels. The graphic below is the process & instrumentation diagram (P&ID) for the blower system courtesy of the blower supplier on the project, Aerzen.
Blower P&ID and control narrative summary (courtesy of Aerzen). Click here to enlarge.
Aeration Blowers Provide Efficiency Gains
Various blower technologies were evaluated but ultimately positive displacement (PD) blowers with twisted lobe/screw impellers were specified due to their favorable turn-down potential (nearly four to one) all the while maintaining constant pressure and a high level of efficiency. A wide range of system turn-down was required to accommodate the diurnal and wet weather variations and increases in organic loading over time as well as Phase II expansion aeration demands.
A bifurcated blower header was designed to establish two separate banks (Bank No. 1 and Bank No. 2) of blowers accommodating a total of six equally sized blowers (1,575 SCFM @ 10.00 psig with 125 HP motors). There were several maintenance advantages of selecting equally sized blowers of the same technology including a reduction of required on-hand spare parts and simplified maintenance schedules.
Blower Bank No. 1 will service the aeration and chlorine contact basins accommodating four blowers (Blowers Nos. 1- 4). The second bank of blowers will service the aerobic digester basins and accommodate two blowers (Blowers Nos. 5-6). Firm capacity is achieved by installing butterfly valves on the discharge header on both sides of Blower No. 4, effectively bifurcating the header, so that it can be switched over to either bank. NEMA 3R blower enclosures were specified as the blowers will be housed in a three-sided building to protect against the elements but also aid in ventilation. The graphic below is the P&ID for the aeration system, courtesy of Aerzen.
Aeration System Control P&ID (courtesy of Aerzen). Click here to enlarge.
Deeper Aeration Accounts for Side Water Depth
The relatively high blower pressure, for the industry and region, was needed to account for to the 20-feet side water depth (SWD) of the proposed aeration basins. A 20-feet SWD was chosen to decrease the footprint of the aeration basins to alleviate site constraint concerns. A sharp crested weir on the effluent side maintains the prescribed SWD with diffuser submergence depth staying relatively constant for all flows. The deeper aeration was also beneficial from an oxygen transfer perspective with the proposed fine bubble diffusers achieving upwards of 37% standard oxygen transfer efficiency (SOTE) at the designed SWD.
One potential drawback of deeper aeration basins is the disruption of the air flow split due to different pressures with other basins with lower SWD. In particular, the proposed chlorine contact basins are designed at a shallower (12-feet) depth. To address this issue, the proposed design calls for an air flow control valve, flow meter, and pressure transmitter at the chlorine contact basin air header to regulate pressure and air flow to meet the minimum mixing and dissolved oxygen requirements.
To accommodate the municipalities budgets for Phase I, only three (3) PD blowers will be installed to serve the new aeration and chlorine contact basins. The existing multi-stage centrifugal blowers serving the converted modular treatment unit into aerobic digester will remain. Although the existing multi-stage centrifugal blowers are at risk for inundation, in an extreme rain/flooding event, the repaired blowers are still in acceptable condition and can still service the treatment module. The additional three (3) blowers will be added when either the Phase II treatment capacity is needed or when until the existing multi-stage centrifugal blowers the reach the end of their useful life or damaged by further flooding.
One aeration design feature of note in the existing 0.95 MGD treatment module is the implementation of single drop coarse bubble diffusers within shear (draft) tubes (Ovivo Enviroquip®) to facilitate mixing and aeration of the deep (20-feet) aeration and digester basins. A shear tube functions essentially as an airlift pump creating differential pressure between the bottom of the tube and the aerated water surface at the top of the tube. Sludge or mixed liquor (ML) is drawn up inside the tube and discharged out the top producing a rolling pattern out from each tube, down the depth of the water, and back up the tube. Overall, the system has the potential to lower blower horsepower compared to a floor mounted aeration system as the diffuser heads are mounted only a portion of the full depth of the tank requiring lower blower pressures due to a reduction in diffuser submergence.
Proposed Reduction of 40-60% in Electrical Costs
The proposed aeration blower design is anticipated to reduce the municipalities aeration electrical costs by 40-60% compared to a conventional coarse bubble aeration system with multi-stage centrifugal blowers with limited turn down. Energy savings are found in the high SOTE due to the increased aeration depth reducing biological treatment air demand along with the automated aeration control system coupled with strategic blower sizing. By selecting multiple with lower capacity blowers (six (6) blowers at 25% design capacity) allows turn-down capability to be maintained as capacity demands increases in the future (as opposed to two (2) blowers at 100% design capacity).
The automated aeration control system is designed to have two (2) dissolved oxygen probes and one (1) ammonium probe installed in each aeration train to send real-time operational data back to the control system. We specified “Most Open Valve” aeration control system for the biological treatment based upon the average dissolved oxygen concentration in the aeration basins. A minimum system air demand will be programmed to ensure the minimum air mixing requirements are met in order to keep solids suspended as ancillary mechanical mixing is not provided in the aeration basins in Phase I.
The proposed ammonium probes are to be installed on the backend of the aeration train to function as a secondary check of the aeration control performance. The control system has the capability to be upgraded in the future to provide cascaded ammonia-based aeration control (ABAC). If biological nutrient removal goals become more stringent in the future, ABAC may represent a further energy savings opportunity over operating at constant user-defined DO set points. Having the ammonium probes report data to the blower PLC can also act as an amplification swing check for the DO control system. Occasionally with the MOV approach, the blowers will modify speed and air flow to accommodate an average DO reading but while the average is being met, the swings from high DO to low DO might cause treatment issues and the ammonium probe can be a trigger for concern that the DO amplitude swing is causing treatment performance issues.
Two aeration control PLCs were specified, one dedicated to aeration basin control and the other for aerobic digesters. The aeration control for the aerobic digester will be able to accommodate future controls based upon either digester water level or dissolved oxygen.
Specifying Responsibility for all Aeration Process Componentes
We specified the blower manufacturer to be responsible for all components of the aeration process including, blowers, control valves, instrumentation, and programming in order to streamline both the installation and maintenance of the system and to allow for ownership responsibility to be held by the equipment experts. We also specified quarterly inspections of the aeration system by the blower manufacturer over the first two-years of operations to provide the facility operator ample training and support in operating and maintaining the system. This is even more important for this facility and client as the facility does not operate with full time staff.
Aerzen blowers were packaged as part of the winning public bid for the Phase I expansion work. Aerzen has local full-service center in the Houston area and is locally represented by Hartwell Environmental Corp. Aerzen’s AERprocess supports both DO and ABAC based aeration control. In addition to biological and blower optimization, the AERprocess also provides both localized and optional remote support of the blower systems. Maintenance schedules and preventative measures are included in the overall control strategy. The customer is prompted to perform system maintenance based on run time or condition monitoring. Optional remote support is provided using a customer controlled 2048-bit encrypted LAN or cellular data connection.
Overall, the proposed design optimized the downsides of the site by providing deeper aeration basins to provide more efficient oxygen transfer which reduced blower sizing, combining aeration systems to reduce building sizing and costs, and design an intelligent aeration control system to capture energy efficiencies and provide operational assurances. A real-life engineering example of “when life gives you lemons…”.
About the Authors
Sam Werner P.E., Assistant Project Manager, Jones|Carter, has worked to improve the energy and operational efficiency for a wide variety of WWTPs in the Houston area. email: email@example.com. Jacob Valentien P.E., Senior Project Manager, Pacheco Koch, has over eight years of wastewater facility design experience in the Houston and Central Texas regions. email: firstname.lastname@example.org. The authors would like to thank Victor Orta of A&S Engineers, Inc., Matt Davis of Hartwell Environmental Corp., and Adam Clarke of Aerzen USA Corp. for their contributions to this article.
Jones|Carter is an award-winning, full-service civil engineering, planning, surveying and consulting firm. With more than 500 employees in 10 office across Texas, our team is organized around 12 services that support nearly 20 public and private market sectors. Our mission is simple – enhance lives through engineering excellence. Learn more at www.jonescarter.com.
About Pacheco Koch
Pacheco Koch offers best-in-class professional design services to the south-central region, with offices in Dallas, Fort Worth, Houston, Austin, and Celina offering full-service civil engineering, surveying, landscape architecture, and consulting services. Pacheco Koch has completed over 10,000 civil engineering, landscape architecture, and land surveying projects with some of the most leading-edge firms and progressive public and private clientele. Explore our portfolio or view our services at www.pkce.com.
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