Retrofitting an Existing Water Reclamation Plant

Retrofitting an Existing Water Reclamation Plant – Important Considerations and Challenges

By Madan Arora, Ph.D., P.E., BCEE


As the need for upgrading and/or expanding the capacity of the existing plants is becoming common all over the country, there are some important considerations that merit keeping in mind so that the upgrades and/or expansions are cost-effective, utilize minimum resources, and are energy- neutral to the extent possible.

The need for upgrades and expansions generally arises for two reasons:

  1. The effluent requirements have changed since the plant was first built: these have become more stringent
  2. The service area of the plant has grown with associated population increase

It is anticipated that the future will see more retrofits to the existing plants than brand new green-field plants for the above reasons. It is therefore, paramount that the existing facilities be used to the maximum extent possible to keep the costs down. The fact that funding for such projects is getting more and more difficult- whether by grants or loans, this requirement is gaining more importance in our planning, design and construction of these facilities.

Issues and Concepts

Figure 1-Capacity analysis of an existing plant in California (three parallel trains at one plant)

The first important question that should come to our minds is “what the existing plant can do?” Sometimes, the existing plant- by simply tweaking operational parameters- can handle more flow and/or produce better effluent conforming to the new stringent requirements.  At other times, by adding a particular unit process, the capacity can be significantly increased and/or the effluent quality can be significantly improved.  For example, by adding a coagulant and coagulant-aid to the primary clarifiers, the primary clarifiers can handle more flow and/or remove significantly more pollutants (BOD, TSS etc.) which, in turn, will increase the capacity of the existing secondary treatment.  To illustrate this point further, it is pointed out that the addition of 5 to 10 mg/l of ferric chloride and 0.2 to o.5 mg/l of a suitable anionic polymer can increase the removals of BOD and TSS in primary clarifiers from 30 to 50 plus % of BOD and from 60 to 75% plus of TSS, respectively. These increased removals will put less load onto the secondary treatment system thus increasing its capacity.

This is only one simple example of determining “what the existing plant can do? “.  In order to expand this concept, it is helpful- if not necessary- to determine the capacity of each unit process based on different parameters. As an example, for this simple case of primary clarifiers, the capacity of this unit process may be different based on detention time (say 2 hours at average daily flow) than if it were based on a specified surface settling rate (say 1000 gpd/ft.2 i.e. 41 m3/m2/day for the same flow).  In that case, the designer will have to decide which of the two criteria is more appropriate in determining capacity (most likely the surface settling rate) before making a decision if additional primary clarifiers are necessary.

Another good example to illustrate this point is in the area of secondary treatment; consisting of aeration basins, aeration blowers, aeration diffusers, and secondary clarifiers.  Capacity of each element here is also typically different.  The capacity of the entire secondary treatment process would, therefore, be limited by whichever element has the least capacity.  For example, the aeration tanks may have adequate capacity, but the blowers may not or the diffusers may not, in which case adding additional blower(s) or additional diffusers may increase the capacity of the entire secondary process.  In an existing plant, therefore, it is prudent to determine the capacity of each unit process individually (sometimes, it may even be prudent to determine the capacity of a unit process based on different criterion as explained in the above example of primary clarifiers) and plot them on a single diagram as shown in Figure 1.  Review of this diagram would then show at a glance as to which unit process or which element of a unit process is limiting the capacity of the entire treatment plant.  This type of process-by- process analysis, then, can be used to pin-point the bottle-necks in the plant and corrected to increase the capacity of the plant. Sometime, it may simply be a hydraulic bottleneck (say a pipe, a valve) which is limiting capacity.

A simple and yet a good example based on a process by process analysis of an actual wastewater treatment plant in southern California, which was a candidate for expansion to twice the existing capacity, revealed to a great surprise of the client, that the existing secondary clarifiers were limiting the capacity of the plant.  This client was earlier led to believe that the entire secondary treatment train had to be expanded (i.e. the aeration basins, clarifiers, blowers and diffusers) when actually all elements of the secondary treatment train, except secondary clarifiers, could handle much more flow (although not twice). Figure 2 illustrates another important point that sizing of the aeration basins and secondary clarifiers – which are integral components of a secondary treatment process- should be analyzed together and not separately. The figure shows that the capacity of the existing secondary treatment can often be increased by increasing the mixed liquor concentration in the aeration basins (say from 2500 mg/l to 3500 mg/l for a specified sludge age which depends upon the effluent quality objectives) and additional aeration capacity if necessary as long as the secondary clarifiers can handle the extra solids load. If they cannot, a simpler and cheaper solution to increase capacity may simply be to build additional clarifier(s) as was determined in the example above.  A diagram similar to Figure 1 can lead to such simpler and cost-effective solutions to achieving enhanced capacity of an existing treatment plant.

This type of analysis can be used for all unit processes in an existing treatment plant to determine bottlenecks and develop cost-effective solutions to plant expansions and/or upgrades.

Important Considerations

There are several considerations which should go into the planning and design of expanding and/or upgrading existing plants. These are:

  • New influent characteristics
  • New effluent requirements
  • Impending area-wide water resources programs and/or migration of new industries into the area
  • Sustainability and carbon foot-print
  • Think future. How far?
  • Good neighborly relations
  • New technologies & processes and integration with existing facilities
  • Costs (capital & O&M)

The following paragraphs illustrate these points.

New Influent Characteristics

It is important to know what the current influent characteristics are before embarking on the design of new facilities. To establish this, the designers should make themselves aware of how the data are being collected, stored and analyzed.  Sometimes, plants have measured in the past TOC or COD and used text book conversion factors to convert these parameters into BOD. It has also been seen that that some plants have used the same conversion factor for the influent and primary effluent.  Both procedures are wrong. The conversion factors are plant-specific and vary from the influent to the primary effluent (generally the ratio of COD to BOD increases from the influent down).  Whereas COD to BOD ratio for raw sewage is about 2-2.5, the ratio would likely be higher as the wastewater proceeds through different stages of treatment.  For the final effluent, this ratio could be 4-6 or higher.  Ratio of BOD to TOC, similarly, also changes along the treatment train; say from 2 in the influent to 0.4 in the effluent.  Also, conversion factors may not have remained constant over the years. Relying on the data so collected, assembled and reported will lead to a wrong design of the retrofitted new facilities if based on that data.  It has also been seen that the operators sometimes are not sure if they are measuring carbonaceous BOD or total BOD. They have perhaps been measuring this parameter for years and have gotten used to the procedure, but not realizing what they are measuring.  Typically, total BOD can be 10 to 25 % higher than the carbonaceous BOD: once again it is plant -specific. It will be prudent therefore for the designer(s) to visit the lab at the plant and ask these questions of the lab personnel. Some plants may be subcontracting the testing services to an outside lab in which case the same concerns should be resolved with that lab. Similar concerns apply to the measurement of other characteristics which should also be addressed (TSS, TKN, NH3 etc.). Starting the design based on wrong influent characteristics is like building a house on a weak foundation.

Several months of representative data (flow and characteristics) should be reviewed and analyzed statistically to determine yearly averages, monthly averages etc. and design based on appropriately selected parameters (typically max month average depending upon the requirement of the discharge permit).  In addition, appropriate process and hydraulic peaking factors should be used based on actual data for determining air requirement (say in an activated sludge plant) and hydraulic conduits such as pipes, channels and valves.

New Effluent Requirements

It is important to know what the effluent requirements are. Study the permit carefully. Also, determine what might be the near-future requirements which may require discussions with the client as well as the regulators who serve that jurisdiction. If the new permit is going to be issued soon, the regulators will likely know and willingly share the relevant information as to what those would be. If the new permit is not likely to be issued soon, it would be prudent to find out what standards the agencies/cities in generally the same locale (e.g. the same watershed) are required to meet at their facilities. Most often, it is easier and more cost-effective to plan- not necessarily design- for these near-future requirements in the current design than later after the facilities are built. This planning could include leaving space for future facilities, providing stubs, blank flanges, knock-out walls and enlarging some pipe sizes in some cases etc. so that future provisions can be made easily without disrupting the operation of newly built retrofitted facilities, or worse having to dismantle/ decommission them after a few years of operation and use.

Impending Area-wide Water Resources Programs and Migration of New Industries into the Area

These include area-wide programs and anticipated new industries that are likely to move in the area which could have significant impact on planning and design.  If the city and/or the agency is considering, on its own or by a mandate as in California, water conservation, it will, undoubtedly, increase the concentration of pollutants. So the data collected in the past will likely be unusable “as is” and will require adjustment. This will particularly affect the design of upstream satellite plants which are typically designed for a certain pre-determined flow capacity dependent on the anticipated water reuse.  It can also affect the design of the main plant, perhaps to a lesser degree, since these plants will receive the same mass loadings of the pollutants; just less flow but at a higher concentration. This should be carefully evaluated.

New anticipated industries-particularly food processing, wine making etc.- can significantly contribute to the flow as well as the concentration of pollutants. If they are anticipated to move to the area soon, it will be prudent to consider these in planning and/or design of the upgrade and/or expansion of the plant.  These industries may contribute flow and pollutants on a continuous basis, or varying from day to day (if they operate only during the week days), or seasonally (if they shut down operations completely or partially in certain times of the year; say an ice cream factory during winter).  Another factor may come into play when a particular industry shuts down production on weekends but cleans their vats and performs other normal house-cleaning activities using chemicals on these days. These chemicals are often biocides, such as quaternary ammonium compounds, which if discharged to the sewer system, can be detrimental to the operation of the water reclamation plant.  The city and/or the agency responsible for operation of the water reclamation plant can, in such cases, require that no such chemicals be used for housekeeping and alternative chemicals be used instead (such as chlorine followed by dechlorinating chemicals such as sodium bisulfite) before discharging the waste to the sewer system. This strategy was successfully implemented in the design of an industrial wastewater treatment plant in central valley California which was required to treat waste from several cheese-making factories in the city (industrial wastewater treatment plant, city of Tulare, California).


“Green” has become fashionable and even necessary in the planning and design of any new or retrofitted facilities. While big industries capture the public attention as likely candidates to minimize environmental footprints (i.e. less energy consumption, reduced consumption of our planet’s resources and water conservation to name a few), wastewater treatment and water reclamation plants can and must do their share in reducing the environmental footprint of their designs. Through easy-to-implement common sense approaches to design and operation, plants can save 30 % or more in power usage, 60 to 80 % in on-site water use, and significant amount of water within the community by implementing water recycling projects. An article by this author on this subject was published in September 2009 issue of the WE&T journal under the title “GREENER PLANTS- Designing and Operating a Sustainable Wastewater Treatment Plant”, which described the areas where reductions in environmental footprints can be easily achieved by careful planning and design of such facilities.

Think Future – How Far?

As pointed out above, planners and designers, with assistance and guidance from their clients, should always consider future in their decision making- both the growth in the service area as well as any changes in the regulatory requirements. The question, however, is how far into the future, consideration is appropriate and wise. There cannot be a single answer to this important question.

Each situation is unique and must be evaluated carefully. A general recommendation is that 10 years’ horizon is prudent. This is because the technologies change, future regulatory requirements are difficult to predict in advance beyond this horizon, and equally important is the question of fairness to the future generation of citizens that the facility in question will serve. Should the future users of the facility pay for any miscalculation- intentional or unintentional- done by the current decision makers since the financial and environmental impacts of such decisions pass onto these users without their consent and input.

Needless to say, each situation is unique and requires deliberate analysis by all the parties involved- management of the city and/or the agency, planners and designers, financiers, and regulators who may be funding, in part, by providing grants, loans and subsidies.

Good Neighborliness

It is being universally recognized now that the wastewater treatment or reclamation plants must be good neighbors. And there is a price associated with being a good neighbor. Should the facilities be enclosed and/or housed? Should the odor emanating processes within the plant be covered with provisions for capturing odors and treating them with modern and state-of-the art odor treatment processes? Should the plant(s) be fully landscaped?  Should they be screened all around from the neighbors with plants and bushes? All these issues should be an integral part of decision making. More and more clients are now willing to pay these extra costs as they are beginning to realize the importance of being good neighbors. This not only accelerates implementation of the projects (no legal challenges) but also enhances the appearance and aesthetics of the project(s): even making such projects as learning centers for citizens, school and college students, and local social clubs such as Rotary, Lions and others.  This is not a “pie in the sky” anymore.  A wastewater treatment plant owned and operated by the City of Los Angeles, California, “Tillman Water Reclamation Plant” houses a beautiful Japanese Garden on its property which is regularly visited by tourists and has become a sought-after place for holding wedding ceremonies and receptions.

New Technologies & Processes and Integration with Existing Facilities

New technologies and systems are being continually developed which aim at reducing power consumption, producing better effluent at lower cost, using greener technologies and/or combination of the above. The planners and designers must have an open mind and continually update their knowledge by attending technical conferences, reading related journals and articles and apply that knowledge to their projects.

Examples of these emerging technologies are many and include MBRs (membrane bioreactors), new filtration systems (such as disk filters), disinfection by UV, ozone, hydrogen peroxide and/or combination of these, digestion of sludges to improve destruction of volatile solids in the sludge and increase pathogen kill, removal of nutrients, water reuse, and reduction/ elimination of residual solids for off-site disposal, to name a few.

Sometimes, integration of the technologies and unit processes existing at the plant and new or newer unit processes under consideration for upgrading and/or expansion can pose a challenge.  For example, in existing trickling filter plants, new permit limits requiring nitrogen removed may necessitate abandoning/decommissioning the trickling filters if an activated sludge plant using MLE process is being considered.  This is because the trickling filters are removing soluble BOD, which MLE process needs as a food for the denitrifying bacteria.  Two plants in the California central valley faced this problem (City of Visalia and City of Bakersfield) where a detailed analysis showed that the existing trickling filters were counterproductive to the removal of nitrogen and hence were decommissioned as a part of upgrading and expansion of these plants.  However, in Bakersfield, the decommissioned trickling filters were converted into bio-filters for odor control which successfully put that unit process for good use.

It is incumbent, therefore, on the planners and designers to keep abreast of these evolving technologies, issues and processes and incorporate them or address them in their designs as appropriate. Only then, we engineers and planners, can claim that we are best serving our clients.

Cost-Capital & O&M

While planning these facilities, costs should not be ignored. While capital cost may be a one-time expense (although it invariably results in annual cost if financed with a loan or loans), the O&M costs are recurring for life of the project. These costs are payable by future generations and hence should receive full careful evaluation before decisions are cast in concrete.


This paper highlights considerations that are important in retrofitting, upgrading and expanding the existing wastewater treatment and water reclamation plants. Existing plants can possibly achieve more in handling flow and/or improving effluent quality by carefully reviewing operation and design and determining and correcting bottlenecks. The newer technologies and processes should be considered as they apply to the project in hand, judiciously providing for the future, designing plants which are aesthetically pleasing and are good neighbors, and providing technologies which are environmentally friendly and GREEN.

A tall order, but nonetheless important and achievable.



About the Author

Madan Arora, Ph.D., P.E., BCEE

Madan Arora, Ph.D., P.E., BCEE is a technical director at Parsons Corp. (Pasadena, California)

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