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Classic Mistakes in Water and Effluent Treatment Plant Design and Operation #2
In this follow-up to my article on design team errors, I list a few errors more commonly made on the operational side. There is more detail on this in my new book, “An Applied Guide to Water and Effluent Treatment Plant Design”, available for pre-order now.
“The Plant Works…Most of the Time”
I see this issue most commonly in troubleshooting and expert witness work. A client will maintain that their plant works, but the sampling data shows that the plant does not always meet specification. It may even be that every sample taken has failed, but the client remains convinced that the plant is working the rest of the time, so it’s working. Mostly.
This may be an operational failure, but it is most commonly an issue of underdesign. The design envelope must be set to have the plant working as much of the time as specified. This is most commonly 100% of the time, though there are exceptions. The costs of each incremental improvement in treatment efficiency may rise exponentially, so underdesign is tempting in as competitive an environment as water treatment.
A plant which has been underdesigned may “work” some of the time, but “working” means meeting specification to an engineer, and the specification usually includes acceptable ranges of availability and product variability. Working some of the time is more properly called “not working” from an engineer’s point of view. Managers and other non-engineers may however have another opinion.
“Water Treatment Plant Design is Easy”
Water chemistry is perhaps less exciting than that of more energetic compounds, and much of the equipment looks archaic. Some would argue that there is nothing new or clever happening in water: we just follow the design manual blindly, price the equipment out of a catalogue, and build it as cheaply as possible.
There is some truth in this. It is indeed possible to build some sort of water treatment plant in this way and I have seen it done several times. Plants like this are inferior in two respects to those designed by experts: cost and robustness.
The water industry is savagely competitive and cyclical. Margins are always very tight, and tendering jobs at cost or even below cost is commonplace during downturns. Water process engineers design themselves ahead, rather than competing with discounters on a like for like basis.
The characterization of the chemical makeup of the feedstock to a water treatment plant is usually very poor, flowrates are variable, and treatability or pilot trials are very rare. Water process engineers have to make best use of experience, heuristics and statistical analysis to generate a number of scenarios which define the design envelope. This approach produces a robust design.
The fallacy that water treatment plant design is easy leads to another: the notion that it is an ideal starting point for a new designer.
“Let The Newbie Design It”
I have carried out several expert witness engagements where fresh graduates or even undergraduates were allowed – by large international engineering firms without in-house water treatment process design expertise – to design water treatment plants.
This may have seemed like a cost-effective decision, but new graduate chemical engineers usually have no knowledge of water chemistry or the unit operations used for treatment. Universities tend to focus on oil and gas or bulk chemical design, to the extent that they teach design at all. Graduates have a very limited knowledge of biology or statistics, and are used only to the reaction chemistry of pure substances. They might be aspiring process engineers, but they are likely to know nothing about water treatment plant design, or even its underpinning sciences and math.
So, don’t let the newbie design it. By the time companies have paid for the correction of their mistakes, they may have spent many times what it would have cost to employ a specialist.
“Why Call in a Specialist?”
In my professional life I have seen many effluent treatment plants designed by experts in the main process rather than in effluent treatment. These plants are often needlessly expensive, lack robustness, and miss the basic tricks of the specialist.
For example, the designs of water treatment specialists will not carry water-containing, potentially pipe-blocking materials in pipes. These blockages, of which the fatberg is a prime example, are the key reason water specialists make far more use of open channels than other process designers.
I have seen non-specialists learn the hard way why open channels are used on more than one occasion. Neither is it a good idea to filter water containing such materials, as I have seen non-specialists try, as this simply deposits a fatberg on the filter.
Water specialists tend to add far more buffering capacity to our plants than non-specialists, to deal with the variability in flow and composition of feed, and the increased possibility of unscheduled maintenance.
Most importantly, specialists know how to account economically for the variability of feed. Non-specialists may try to surmount this issue by simply making everything much bigger than it needs to be, but many items of plant can be too big as well as too small. Turndown ratios matter a lot in water treatment plant design.
“Who Needs a Process Engineer?”
Those cases I have been involved with in which a process plant was built without process engineering input are useful to demonstrate what process engineers do and how we add value to a project despite being expensive.
I have seen scientists and civil engineers called process engineers in some organizations. A job title does not make a scientist into an engineer, or give a civil engineer the required skillset. Process engineers must have with proven skills in the areas outlined above to carry out a process design.
“Ye cannae change the laws of physics, Jim”
It is important not to be seduced by the claims of novel processes and their associated marketing breakthroughs, such as zero-sludge-yield assumptions.
Any process at lab scale fed with a constant flow of water of constant composition will exceed the performance of full-scale commercial water engineering equipment by a large factor. As discussed in the previous article, this is not meaningful. Our design heuristics include very significant comfort margins. We will need strong evidence at full-scale before even considering changing them.
Did I mention that my book includes these (previously unpublished) design heuristics? And that it is available for preorder now? Fill your boots!
About the author
Professor Sean Moran is a Chartered Engineer with over twenty years’ experience in process design, commissioning and troubleshooting and is regarded as the ‘voice of chemical engineering’. He started his career with international process engineering contractors and worked worldwide on water treatment projects before setting up his own consultancy in 1996, specializing in process and hydraulic design, commissioning and troubleshooting of industrial effluent and water treatment plants.
Whilst Associate Professor at the University of Nottingham, he coordinated the design teaching program for chemical engineering students. Professor Moran’s university work focused on increasing industrial relevance in teaching, with a particular emphasis on process design, safety and employability.
Sean’s latest books are also available to order on the Elsevier Store. Use discount code STC317 at checkout and save up to 30% on your very own copy!
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