Designing a treatment train

Consultant perspective on the design process

From the consulting perspective, what normally happens is some water quality expert already knows the top 3 - 5 technologies or combination of technologies that are most appropriate for the water being treated. Then we start doing a deeper dive into different vendors that offer those technologies, talk to the vendor representatives about what their technology can do, if there are any concerns that it won't work given the starting water quality and the requirements that need to be met, prices, maybe piloting. A consulting practice would never really being trying to figure out which of the 10+ treatment technologies is a good fit, it would be narrowed down to 4 - 5 different technologies (and probably at least two would be used in conjunction with each other) pretty fast. Then the next step is really manufacturers/vendors.

The following is from John Erickson in response to my question about what makes design of water treatment plants challenging and interesting.

1. Every situation is a little different. So even if the processes and equipment are out there "on the shelf" it still takes quite a bit of engineering to select the correct ones and make sure they work well together. For example:

   • Plants in the U.S. are often designed to manage quite a few water quality parameters in addition to turbidity and microbes, both at the plant and in the distribution system (examples below).
     • Disinfection byproducts (DBPs)
     • Corrosion parameters
     • Organic carbon -- which will affect disinfectant residual stability and the potential for nitirification or DBP formation in the distribution system
     • Taste and odor
     • Heavy metals
   • Source water can also complicate treatment (algal blooms, high TOC, high TDS or heavy metals) *Depending on source water, water quality goals, water quality issues you can expect in your distribution system (based on residence time and temperature), additional or enhanced treatment processes are often necessary to meet water goals:
     • GAC or ion exchange to control DBP precursors
     • Aeration, GAC or PAC for taste and odor
     • Enhanced coagulation to deal with TOC
     • Ozone + Biologically Active Filtration for all of the above
     • RO treatment of all or part of the flow to reduce TDS
     • Aeration out in the distribution system to strip DBPs
   • Brand new, built-from-scratch WTP design projects seem to be pretty rare, even here in Texas. Most projects are expansions, improvements, rehabs, etc. So the engineer has to design new processes to fit in and coordinate with existing infrastructure. The engineer normally also has to figure out how to build the new infrastructure while the existing infrastructure remains in operation.
   • With all of these details, figuring out how to meet treatment goals, minimize cost, and end up with a plant that will be usable and expandable into the future can be a tricky optimization problem.

2. I agree a lot of the effort and complexity of WTP design in the U.S. is related to all the ancillary parts (SCADA, controls, hydraulics, chemical containment, safety, redundancy, etc.) rather than the actual fundamental processes. However, to design those bells and whistles well you need a good understanding of the processes that the bells and whistles are supporting. To design instrumentation and controls for a process, you need to know what can go wrong with the process, what the indicators of something going wrong are, and what the operator can change to get things back on course. The actual design of the instrumentation and controls will probably be done by someone who specializes in instrumentation and controls for WTPs. But the process engineer will still have to provide a lot of oversight.

3. Like most engineering projects, the non-technical parts (see a few below) require as much if not more effort than the technical parts. But it normally requires strong technical skills to do these non-technical things correctly. This non-technical stuff is probably better learned after school than in school, so maybe shouldn't be a big part of an engineering design class. But I think including a couple lectures on it would be helpful as background and so students see what they could be getting into career-wise. My understanding of some of this was slim to none before I started working at Hazen. Most water and wastewater projects in the U.S. are "design-bid-build", meaning the engineer develops plans and specifications that then serve as a contract between the utility and the construction contractor who is the lowest bidder. A lot of thought and effort goes into making plans and specs that are general enough to result in a competitive bid (multiple manufacturers can provide key equipment, for instance) but specific enough to make sure a the contractor building the project isn't allowed to sacrifice quality for cost.

   • Working with the client (who is typically busy and consists of multiple stakeholders -- engineers, operators, managers) to develop a design they're happy with, meets their budget, and will work.
   • Coordinating with all the non-process engineers required to build a WTP (electrical, structural, surveyor...)
   • Getting the design permitted by the state regulator
   • Getting all of this done efficiently to stay within the design budget for the project

Reflections on treatment train design (Michael Adelman and Tori Klug)

In many cases, what the process will be is known in advance. Projects like rehabilitating media filters, installing a CO2 feed system for pH control, etc. get right into sizing and detailed design because we know from the beginning which process will be used. That said, “what contaminants are we dealing with and what is the best process to treat them” is a question we’ve had to deal with a number of times, and typically the best answer is found by doing some preliminary sizing and cost calcs. In other words, great teaching material!

Examples

• Groundwater with high VOCs, metals, and 1,4-dioxane à treatment train of air stripping, ion exchange resin, UVAOP (with hydrogen peroxide dosed), GAC. Could discuss tradeoff of air stripping capacity versus GAC volume/changeout frequency for VOC removal (VOCs will also be destroyed in UVAOP, which is required for 1,4-dioxane removal). GAC would need to be used to quench peroxide residual if a drinking water application, could also discuss removing GAC (and thus relying on air strippers and UVAOP for VOC removal) if quenching of peroxide residual is not necessary (e.g., discharge to sewer). Could remove air strippers and rely on GAC and UVAOP for VOC removal. Could be interesting to make up concentrations for some VOCs to show when one process might be more appropriate than the other, and how individual VOC Henry’s law constants and adsorption parameters can play a role in determining which treatment process is preferable and in governing sizing/media changeout. Could also present a scenario in which you just have VOCs and treat using air stripping or GAC, then find 1,4-dioxane and need to add UVAOP and determine if air stripping/GAC will still be necessary after accounting for VOC destruction in UVAOP.
• Groundwater with high arsenic and rising nitrate levels à arsenic adsorption filters and/or RO. Could present different concentrations and think through tradeoffs of each process (e.g., differences in capital and O&M costs) at different concentrations of arsenic and nitrate, and see when RO will be necessary to remove nitrate given groundwater quality projections.
• Chino Desalter Authority VOC studies. The Chino Desalter Authority operates two RO plants treating brackish groundwater. Some of their wells are impacted by various VOC plumes, and we did a few studies for them about how best to address TCE and 1,2,3-TCP with some combination of their existing treatment systems and new potential processes. Content includes adsorption, stripping (i.e. repurposing decarbonators), advanced oxidation, and RO.
• Las Virgenes and Santa Monica water reuse studies. We did some conceptual-level studies a few years ago that led to ongoing potable reuse projects. At Las Virgenes, water recycling helped them solve a water supply/storage problem and our treatment contribution was to flesh out Full Advanced Treatment (MF-RO-AOP), and for Santa Monica we evaluated stormwater treatment upgrades AND potable reuse as a way to achieve more water independence.
• Stormwater and potable reuse are hot topics and might be good to include in the class.
• Pure Water San Diego could be a good example here – extra treatment processes for closer to direct potable reuse!

If John Erickson were making a WTP design project he would have it be a modification or expansion of an existing facility and try to include these 3 components:

1. Data analysis:
   • Drawings of existing plant you're modifying
   • Source or finished water quality data (at least a year of it) relevant to whatever you're trying to design
   • Any relevant SCADA data for hydraulics, plant flow, etc.
2. Alternatives analysis:
   • Develop at least two alternatives (these could be different processes, or hydraulic configurations, whatever is relevant)
   • Compare them based on performance, cost, and impact on operations.
   • Write a short memo with comparison and recommendation of which to select
3. Design and Documentation:
   • Short design report defining design parameters (capacity, footprint, headloss, power requirements)
   • Simple drawing - Maybe 1 plan and 1 section view
   • Specification for one component of whatever you're designing - a pipe, equipment, valve, whatever. They could just pull a spec off the internet and modify it to fit their project. Might be neat to find one multi-faceted project and have different students teams work on each aspect. Then they could have coordination meetings to figure out how their processes will connect and whether there will be enough room to fit them all on the site.