Designing Cost-Efficient Wastewater Treatment Systems

Are You Overdesigning and Overspending for Your Wastewater Needs?

The cost of wastewater treatment will almost always result in some sticker shock, hand-wringing and head-scratching by both municipalities and developers. How should we, as engineers, approach the task of reducing costs while not sacrificing reliability, performance, and energy efficiency?

Among the approaches to consider, in order of their place in the design process, are the following:

• Are the treatment standards for your region still applicable in the current age?
• Are you using the most current proven technologies?
• Are you using the most suitable materials and equipment?
• Are you considering life-cycle costs in the selection of materials and equipment?
• Are you preparing your designs in the most efficient manner? … and including a “What-If” analysis?
• Are there dangers of overdesign that don’t include cost?

It must be emphasized that the issue is not just that we are overdesigning and overspending; wastewater treatment plants are often crippled and unable to perform properly because they are overdesigned. In the first iteration of this white paper, the focus is primarily on the first bullet above, “Are the treatment standards for your region still applicable in the current age?”, as it is currently the most prominent of the issues identified. Although I will briefly touch upon the 2nd and 3rd bullet, a subsequent publication will provide greater details on those.

Many of the treatment standards in use today were first developed almost 100 years ago. Do they still apply today? Should they? It has long been this writer’s professional opinion that the answer is, at best, a big “Maybe”. In my experience over the last 4+ decades, design standards in certain jurisdictions typically result in wastewater treatment facilities being built at twice the size they need to be. This is especially true with respect to smaller package plants serving newly proposed / planned communities where existing flow data cannot be obtained. In the schematic design below, experience has shown that facilities like this will typically be operated by putting just one flow train in service, with the Plant Operator switching between Flow Train No. 1 for six months and Flow Train No. 2 for six months.

History

The discussion of Wastewater Treatment Design Standards is a tough subject to broach without first recognizing the history of such standards.

While wastewater collection systems existed in early Rome, the earliest mention of wastewater treatment comes from ancient Greece around 1500 BC. Here in the United States, the 1880s standard for wastewater treatment was an outhouse or even an open ditch. In 1860, the “sewered” population was about 1 million people, by 1900 that grew to 25 million. The treatment at that time, if you could call it that, was dilution. The phrase “the solution to pollution is dilution” is still uttered today and it involves basically removing wastewater from areas of dense population to “receiving waters”. This is great for the people upstream, not so much for those downstream. However, it was most often diluted to the point where no one took much notice.

If you would like to expand your understanding of the history of wastewater treatment in the United States, refer to the link: US Wastewater Treatment History

Development of Modern Era Standards

Larger cities began to develop and the first major regulatory action in the United States came with the Federal Water Pollution Control Act of 1948.

Biological treatment processes dominate the wastewater technology alternatives, and all of them are based upon the proverbial “what a bear does in the woods” and how nature makes it go away. Exposed to air, natural bacteria in the air break down the waste materials by feeding on the “food source”, using the air to oxidize the organic waste and reduce ammonia (NH₄) into nitrates and nitrites (NO₂ + NO₃) via an aerobic process. As rain falls and washes the remaining waste products into the soil, an anoxic or low-oxygen-level (0.5 mg/l) process takes over.

Today, every biological treatment process mimics what occurs “in the woods” (ITW).  Because the population density in our cities is far greater than “ITW”, it’s the engineer’s job to provide treatment efficiency levels that greatly improve the contact between the biological organisms (biota) and the waste products, as well as provide the proper aerobic / anoxic / anaerobic environment necessary to produce optimal treatment efficiencies.

Early in the 20^th century, research mainly at universities, provided the necessary data to develop design guidelines. A combination of uncertainty due to limited data, as well as the large amounts of water used to flush waste into sewer systems, led to a high level of conservatism in the design standards. For example, early toilet tanks used as much as 7.0 gallons per flush, but even before the 1995 Plumbing Code, most toilets had dropped to 3.5 gallons per flush. That was a reduction of 50%.

Current Standards

The current norm in the industry is still between 75 and 100 gallons per day (GPD) per person. In the first decade of my career, when the federal (85%) and state (10%) governments were footing most of the bill, I participated in the design of several new plant designs, but it was not till the mid-1980s that an opportunity came along to design a plant upgrade. It was for a private condominium community with a permitted capacity of 95,000 GPD using the extended aeration process and it had been mandated to upgrade from their current extended aeration secondary treatment process to provide tertiary treatment.

At that time, the only process acceptable to the local regulatory agency was to add denitrification via sand filters. We reviewed the records, inspected the facilities, and determined that adding “denite” filters would require not only additional tankage, but also a building extension at a cost estimated at $1.25 million. We investigated potential alternatives by taking a two-pronged approach:

• The only tertiary process accepted at that time by the local regulatory agency would require the addition of a new anoxic sand filter system downstream of the existing aerobic tank. It was that additional tankage requirement that necessitated the building expansion. We looked at alternative treatment processes and determined that, by using an alternative treatment technology not currently accepted at the time, we could significantly reduce the building footprint. With the Sequence Batch Reactor (SBR) process, instead of having an aerobic tank followed by an anoxic tank, the SBR simply alternated periods where process air was on, and air was off. However, that still wouldn’t fit inside the existing building as we still didn’t have enough tankage for 95,000 GPD.

• After studying flow records from the plant flow meter and comparing them with flow records from the local water supplier, we determined that the actual flow of the plant was only between 50,000 GPD and 60,000 GPD. We therefore proposed a reduction in capacity. The combination of using an alternative process and a 30% reduction in plant flow eliminated the need for a building expansion. The modern SBR process originated in Australia, and, as of that time, only a few plants had been built in the United States. Our then-estimated cost of conversion was just 20% of our estimated cost for the currently accepted process.
  
• When the HOA Board subsequently conducted a “Solicitation of Proposals” process, feedback from other responding engineers, local and state regulatory agencies resulted in concerns about the reliability of the locally “unproven” process.  Another firm was selected for the design, followed by the approval of plans and specifications utilizing currently approved sand filter technology.  The resulting low bid came in at $1.4 million.  While that was occurring, we began submitting proposals for other plants as, in total, 21 plants had been mandated to upgrade.
  
• What we began to observe was that all the plants which we examined had actual flows much lower than their SPDES permitted capacity.  We subsequently completed engineering design reports for 17 of the 21 cited plants, including the one we originally “lost” to the other firm. We completed that upgrade at a total cost of just $175,000, just 12.5% of the original bid price. The actual design flows we used for those 17 SBR upgrade designs ranged from 33% to 54% of their original design flows. Over the next 35 years, we designed 20 wastewater treatment plant upgrades and about as many new plants. Rarely has any of the plants ever experienced actual flows more than 50% of the design flow.
  
• When the 1995 Plumbing Code mandated 1.6-gallon-per-flush toilets, we increased our efforts “lobbying” for revised standards that reflected actual observed data over the preceding 10 years. The perfectly logical counterargument was “while the amount of water being flushed has decreased by half, the waste material going down with it has not. Therefore, if one takes half the water at twice the concentration, we still get the same biological loadings.”

That was both an obvious and reasonable conclusion as it would apply to biological treatment, however it was also applied at the time to pumping stations.  In every plant / pump station design with which we have been involved in since 1985, to avoid the hazards of overdesign, we have approached the issue from the perspective of “making it work per the regulatory standards” as well as making it work at flows and concentrations that we actually expected to occur.

This approach was based upon referencing the data from past projects, which had been periodically monitored since design and construction, and making equipment selections which allowed for proper and efficient operation throughout the equipment’s entire operational range. Though “logically puzzling”, we rarely obtained influent sample concentrations with a higher influent concentration than that which had been customarily observed before the 1995 Plumbing Code, despite the lower flush volume.

Another factor which comes into play with regard to overly conservative flow standards was the fact that, in addition to what leaves the building through the building connections, engineers have also had to account for infiltration and inflow. As piping technology has improved, the industry has moved from hollowed out logs, to clay, to cast/ductile iron, to jointed PVC and now, to thermally welded HDPE. Outside of combined sewer systems, wet weather flows have almost become a complete non-factor in the design of modern-day facilities. Over the last 35 years, both practicing engineers and efforts by engineering societies, have had little success in obtaining reductions in applicable standards. However, there is light on the horizon.

• When connecting to or upgrading an existing plant, most standards now permit the use of metered flow rates from the community based upon 2-3 years of historical plant data and winter water consumption rates. There is a bit of irony here. We had upgraded another existing pre-1995 Plumbing Code plant to tertiary treatment and confirmed a per-capita flow rate of just 45 GPD per capita (gpcd). We were able to perform the upgrade to tertiary treatment without expanding the building by using the now well-accepted SBR process.

• When a new community was planned to be built on the adjacent property, a literal “mirror image” of the existing facility, we were asked to design a new expanded treatment facility to accommodate both communities in 1996. Our analysis showed that we didn’t need to expand, but the design did not proceed as originally conceived.

• The existing community with 3.5-gallon-per-flush toilets and no water-saving fixtures, had an already approved per-capita flow rate of just 45 gpcd based upon historical data. Our position was that with 1.6-gallon toilets and all other fixtures being 1995 Code-Compliant at the new development, we should be able to use even a smaller flow than the 45 gpcd flow at the older facility with 3.5-gallon toilets. However, since we had the necessary tankage capacity, we thought “why not be conservative and use 45 gpcd”. That proposal was not accepted because there was no “proven history” for the yet unbuilt facility was then based upon the following:
  
  • Existing community with 3.5-gallon-per-flush toilets and no other water saving fixtures could be designed based on 45 gpcd.
  • The new community, a literal mirrored carbon copy of the original community with the same bedroom count, same square footage, and a complete set of 1995 Code-Compliant toilets and fixtures would be designed based upon the still-current standard of 75 gpcd.

While that decision generally raises a few eyebrows, it must be recognized that there is more than science and engineering here – there is regulation. The standard, adopted by the regulatory agency, is based upon legislation or delegation of authority. A local county or municipality serving as the reviewing agency is often delegated that authority by the state and, in turn, the state has “officially” adopted a standard prepared and published by a regional authority. Change is slow; to effect change, engineers must be active in collectively lobbying both individually and through our member societies.

Current Activity

I began to wonder if I’d see it in my lifetime, but things have been changing here, at least locally. Back in 1972, on-site septic systems were subsequently required to include septic tanks and new treatment facilities had to provide tertiary treatment. In the mid-1980s, all existing secondary treatment facilities over 30,000 GPD in size were mandated to upgrade. In the last year, any new or existing buildings expanded or undergoing a change in use, if not served by a treatment facility, must install an Innovative/Alternative On-Site Wastewater Treatment System.

Local elected officials and regulatory agencies, especially in suburban areas, are taking note of the environmental impacts of continued development, to my mind, for three major reasons:

• Much of the suburban areas remain unsewered and existing on-site systems, and even new I/A IOWTS systems, will continue to have a negative impact on groundwater, lakes, streams, and rivers.
• These communities could be connected to the existing neighboring facilities at far less cost than building new facilities or installing advanced on-site systems while providing far greater treatment efficiencies than advanced on-site systems.
• The existing treatment facilities, being overdesigned, are not only inefficient but performance is often impacted due to equipment operating outside its recommended range of service.  By providing more flow, equipment would now be able to operate within its normal operating range.

While regulatory changes in the last 50 years have made significant progress in addressing treatment efficiency, there has been little adjustment in flow rates based upon types of usage.  We haven’t varied much from that 75 –100 gpcd per person contribution.  The following combination of factors, I believe, provides a sound, justifiable basis for a reduction of stipulated flows in current design standards:

• Home renovations over the last 25+ years have to a large part eliminated old high flow water fixtures.
• Improved pipe technologies have negated the impact of infiltration and inflow.
• PLC-based control of pumping and treatment systems has allowed operation over wide flow ranges.
• Lifestyle changes with households having multiple wage earners, has reduced residential water consumption.

A local agency, in our neck of the woods, recently issued an RFP for engineering services after studying flow rates and concentrations at a substantial number of local, privately-owned treatment facilities. When combined with our previous in-house efforts, we now have doubled the data set of plants analyzed in this manner.

Their Phase I conclusion was that the 20 plants studied averaged about 50% of the original design capacity. This mirrors the conclusions drawn in each of the individual facility analyses which we have conducted since 1980. An RFP was issued calling for a Comprehensive Engineering Report addressing how the excess capacity of these existing treatment facilities could be used to provide wastewater treatment for surrounding communities.

Going Forward

Regulatory standards are generally written for the reviewing agency to provide “rules of thumb” for checking a design. The Ten States Standards states the following:

“When process design calculations are not submitted [by the engineer], the aeration tank capacities and permissible loadings shown in the following table shall be used for the several adaptations of the processes …”  All too often, these standards are interpreted as “hard rules” even when validated process design calculations are submitted. With the conservative, artificially inflated flow rates, combined with overly conservative loading rates, we find that plant equipment operates well outside its recommended operating ranges and far from its best efficiency point.Often equipment may have to be disabled to allow the process to meet permit requirements.

Now we come to the part that discusses what we can do about it. As was shown in the previous section, regulatory agencies are taking note that, at least for residential communities, the design flows, in most standards, are grossly exaggerated when used in the modern era. However, this acceptance, with regard to residential communities, loses traction in the commercial sector. For a new facility such as restaurant, office building or industrial building, the concern is a legitimate one. “What if there is a tenant change?” Can existing data from properties of “like kind” be used for new facilities?

The obvious commercial-use candidates are franchise operations, typical office buildings and such, and the assumption is that operations of the same type and size should produce the same results regardless of location. But that won’t always be the case. For example, a fast-food operation in a midwestern rural farming community with little nightlife should not produce the same amount of wastewater as the same type and size of operation in a college town. At this point, I think everyone reading this sees the obvious – that a college town will have a population more inclined to eat fast food and, I expect, more inclined to binge on fast food in the late evening hours.

But as engineers we can foresee this. Another example is an office building where the office is originally designed to be occupied between 9 and 12 hours a day. Will the wastewater system be capable of adapting to a tenant change to a support or call center with 24/7 staffing? The short answer is yes, it can be. Offices are generally in urban or suburban centers where a significant portion of the population has its own existing wastewater collection system. At almost every pump station we have inspected and analyzed, the pump is vastly oversized and operating well outside its recommended range.

In another instance, we recently worked with an out-of-state entity where design standards stipulated an average daily flow of 6,300 GPD for a temporary building which would be utilized while an existing building was renovated. When flow data from the existing buildings past two years of water bills was analyzed, it was only a small fraction of the flow mandated by the current standard.

Conclusion

If you are put in a position where there is a concern that equipment meeting design standards will operate outside of its recommended range, operate inefficiently, or operate outside recommended cycle times, there are alternatives. In lieu of the standard “one operating and one spare” standard, consider that your actual flow might only be about 50% and select a pump that operates at one-half of your design flow and provide for three of them.

If the standards are right, you will meet the design flow with two operating and one spare. If you’re right, you will realize longer pump life, operation within the recommended range, lower utility costs and no cycle time concerns. This strategy can be applied to aeration blowers and most equipment at wastewater treatment plants. With most plants, there is a requirement to always have two flow trains so that if one is down, the other can provide at least partial treatment. In reality, as often as not, one flow train is taken out of service because the aeration equipment is oversized when only receiving half the flow it needs to operate properly.

On one project back in 2020, we were called in by a client who had received bids on a small wastewater project with a low bid of $1.4 million; they asked if we could do anything to make it less expensive. We met with them and told them we could modify the design and bring it in for $750,000.

Before we even got back to the office, the client had called and advised us that the low bidder had called, said they made a mistake, and that their bid would be only $1.1 million dollars. The client asked if we were confident in our numbers. After answering yes, we redesigned the job for about $4,500; then the project was rebid with no General Contractor (GC) with our firm providing Construction Administration of the four subcontractors (Excavation, Plumbing, Concrete and Electrical) as well as the Package Plant Supplier.
 
By providing the Construction Administration, we saved the typical 10% Overhead and 10% profit (O & P) normally charged by the GC for administering the subcontractors. That O & P typically includes a 20-30% markup for the check written for the two package plant flow trains and accessory tankage which are delivered preassembled and dropped into the excavation. Unfortunately, we didn’t quite hit our target of $750,000; the project came in at $758,000.
 
Be sure to check-in with future publications, elaborating on overdesigning and overspending on your wastewater needs.

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