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Classic Mistakes In Water And Effluent Treatment Plant Design and Operation
Process engineers from other sectors can find it difficult to assimilate our often-nebulous design specifications and the vague and sometimes rather flexible nature of the “substances” we work with. Many of these “substances” are more accurately viewed as test results than chemicals.
“Suspended solids” can (unlike matter) be effectively created and destroyed in an effluent treatment plant. Chemicals can also switch from suspended to dissolved solids if the water chemistry changes, with no loss of matter.
Another example is Biochemical Oxygen Demand (BOD). Although specialists discuss it between themselves as if it were a chemical, BOD isn’t really a chemical substance at all, but the amount of oxygen a complex mixture of substances will absorb under given physical, chemical, and biological conditions in a given time.
Also, buffering effects in natural waters make prediction of the amount of acid required to give a specified pH change something of a black art.
Many water and effluent treatment processes are simply too complex for them to be sufficiently well characterized to allow a traditional chemical engineering approach, a fact which is not well understood in academia.
Much academic research in water and effluent treatment is not merely highly speculative, but is based on a misunderstanding of the basic constraints of water and effluent treatment.
It may for example be the case in future that higher regulatory costs to discharge nutrients to environment make the economics of recovery more viable. It will, however, never be economically viable to operate 100% recovery of all components of an aqueous waste stream, as each successive increment in recovery towards 100% costs more than the previous one. Engineers must always consider the balance of costs and benefits.
By this logic, any research based upon a supposed need, say, to remove amounts of trace organics only detectable by means of the most sensitive lab tests which have not been shown to have any effects on humans or environment is probably entirely spurious and a waste of public funds.
On the other hand, nothing less than an entirely robust technique for disinfection will ever be suitable in a water treatment environment, though the reasons for this differ. Clean water treatment must be an entirely reliable source of safe drinking water, even when operated non-ideally.
Sewage and effluent treatment plant designers may in future be given slightly more flexibility with respect to absolute reliability, but the industry is incredibly price sensitive and conservative. Industrial effluent treatment plants are frequently operated by the cheapest staff available, and/or ignored until they break down entirely. Effluent also contains a variable mixture of grit, grease, harsh chemicals, textiles, and other components which block, blind, abrade and corrode anything they meet.
Equipment and processes which need treating with great care are fundamentally unsuitable for water treatment plants, a fact which is ignored by most researchers. I can illustrate this point with two anecdotes from personal experience.
I was once an advisor to a program promoting the use of novel anaerobic treatment designs in industrial effluent treatment. The promotional materials I was issued with have been proven over time to be wildly optimistic, and since then I have been professionally instructed on more than one occasion to report on why these technologies subsequently proved not to work as well in practice as they had in the lab.
My second anecdote is based on a conversation I had with a researcher about a new nanopore diffuser he had designed for aerating sewage. His tests were proceeding well with simulated sewage, as would be expected: the smaller the bubbles, the better the mass transport. Of course, as soon as the nanopores met real sewage, the diffuser didn’t diffuse any more. That wasn’t the only problem – the tiny bubbles didn’t rise as quickly as the larger ones normally used, so they carried over into the next stage, and didn’t stir the aeration basin to keep the solids in suspension. Nano-stuff was, however, the flavor of the moment for funding bodies. As a result, an obviously unsuitable but fashionable technology was investigated purely to attract research funding, rather than because it had a realistic chance of success.
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.
About the book:
An Applied Guide to Water and Effluent Treatment Plant Design brings together the design of process, wastewater, clean water, industrial effluent and sludge treatment plants, looking at the different treatment objectives within each sub-sector, selection and design of physical, chemical and biological treatment processes, and the professional hydraulic design methodologies.
- Explains how to design water and effluent treatment plants that really work
- Accessible introduction to, and overview of, the area that is written from a process engineering perspective
- Covers new treatment technologies and the whole process, from treatment plant design, to commissioning
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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