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What is Special About Water Treatment Plant Design?

By: , Posted on: August 10, 2018

Picture c/o: Lalupa

My new book “An Applied Guide to Water and Effluent Treatment Plant Design” is available now. It is the book I wished were available when I started as a process engineer specializing in water. So much of what you need to know to be a good water engineer was not in the public domain prior to the publication of my book, despite the long pedigree of water engineering.

How long? Well, the picture is of a bronze Roman hydraulic pump and adjustable nozzle dating from 1-200AD, found in a mine in Huelva province, Spain, and the Romans were far from the first water engineers. Whilst there have been impressive feats of water engineering for millennia, the modern forms of both dirty and clean water engineering were pioneered in the UK.

Historically, civil engineers tended to be responsible for the design of municipal water treatment plants, with assistance from chemists for process development research. Civil engineers specializing in this area were known as “environmental” or “sanitary” engineers.

This was the model as far back as the development of the activated sludge process for sewage treatment by Ardern and Lockett in 1915, and the first use of chlorine for drinking water disinfection around the same time.

Since the days of Ardern and Lockett, we have moved from design by a combination of chemists and civil engineers towards integrated process design by chemical engineers. A colleague at a university where I teach water treatment plant design to civil engineers confided recently that the dirty secret of environmental engineering is that everyone who teaches it nowadays is a chemical engineer.

There are many substantial and longstanding textbooks on water treatment plant design. Prime examples include Metcalf and Eddy for wastewater and Twort for clean water supply. These books do however tend to have been written both by and for environmental engineers, whose focus is generally not on detailed process design. They can consequently lack much of the information which chemical engineers need to follow their rather different approach to process design.

Interactions between chemical process engineers and other engineering disciplines can be slightly complicated because we have traditionally not had a role in water treatment. Nevertheless, there are reasonably clear expectations on the documents which process engineers produce for other disciplines, most notably electrical, civil and software engineers.

The electrical engineer will need process engineers to produce schedules of drives, actuated valves and instruments, together with a layout drawing showing where drives and instruments are located.

The software engineer will need the functional design specification (FDS) and the piping and instrumentation diagram (P&ID) from the process engineer to allow them to price the control software and hardware.

The civil engineer will need layout drawings, marked with the weights of any major items to be placed on the slab. If concrete water-retaining structures are being used, they will need to know their internal dimensions and what they are to contain. They may also be relying on the process engineer to comment on the chemical nature of fluids in contact with concrete from the point of view of concrete specifications.

Water treatment, especially dirty water treatment, is unusual in making extensive use of open channels, weirs, flumes and open-topped vessels. This requires “open channel” hydraulic calculations which may be unfamiliar from academic fluid mechanics courses, or professional practice in other sectors.

As a schoolboy I always thought water chemistry to be the least exciting kind, mainly because of the low potential for explosions, but it is actually an extensive field, about which I am still learning new things after twenty-five years of practice.

Water chemistry tends to be rather neglected nowadays, both at school/college and university level, though it is of great importance in biotechnology and food processing as well as in water treatment. Other engineers will however usually look to the chemical engineer in the water sector for a detailed understanding of water chemistry.

My own background is in applied biology, so I am lucky to have a good grounding in the biological underpinnings of water treatment, often even less well understood by engineers than water chemistry. It is common for both chemical and civil engineers to struggle with the design of biological processes for water treatment plants.

Biochemical engineering allows a rigorous mathematical and chemical analysis of a monoculture of organisms held under controlled conditions and fed with pure chemicals. However, such approaches are of little use in dirty water treatment. Effluent treatment plants contain thousands of types of organisms in variable proportions under uncontrolled conditions being fed with thousands of different compounds in variable proportions. A balance must be therefore struck between, on the one hand, coarse rules of thumb applied without much understanding and, on the other, an unrealistically precise calculation.

In water treatment, it is critical to identify the cheapest way to meet the specification safely and robustly, to the extent that robustness and even safety may be underemphasized by some designers. I was amused to read an academic research paper recently claiming that all the problems of total closed loop water and nutrient recycling had been solved, except for the economics. Economics are, in most cases, the only significant problem in water treatment. Even the dirtiest, most polluted water can be made into the cleanest, if cost were not an issue. (Persuading people to drink the product, however, may still be a problem, for a variety of reasons).

Water treatment engineering usually operates at close to ambient temperature, and at low pressures. Aqueous solutions are often the most aggressive chemical used. Water engineers must almost always be looking for the lowest capital cost solution. This has a strong effect on our material selection choices.

Whilst the bare carbon steel favored in hydrocarbon based chemical engineering may be relatively cheap, it is readily corroded by all but the most highly treated water. The only time I have seen it used in water treatment plants is when a non-specialist has been allowed to design them.

If water specialists use ferrous metals, they tend to be either stainless steel or, far more commonly, the much cheaper cast/ductile iron which is significantly more resistant to corrosion than carbon steel. If we use mild steel, it tends to be coated with something such as zinc, (galvanized mild steel or GMS), glass (enameled, which we call “glass-on-steel”), epoxy, chlorinated rubber, etc.

Specialists make significant use of thermoplastics, such as polyvinyl chloride (PVC) and acrylonitrile butadiene styrene (ABS), especially for piping systems below 300mm nominal bore (NB). Plastics are strong, light, cheap, and do not corrode. Some need to be protected from light and heat, but this is straightforward and inexpensive.

We also use copper, brass and bronzes far more frequently than ‘traditional’ process engineering sectors.

These differences aside, water process engineering is chemical engineering nowadays. Though academic chemical engineers may insist it is civil engineering, academic civil engineers are clear it is not.

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.

Key Features:

  • 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!

Read more articles from Sean Moran, The Voice of Chemical Engineering

Professor Sean Moran is a Chartered Chemical 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.

Connect with Sean on LinkedIn here, check out his Facebook page here and stay up-to-date on his thoughts, research and practice at his personal blog here.

 

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