In the university education, we are taught engineering designs with first principles.
In practice, we rarely attempt to design units applying these principles that we learn from the textbooks. This is probably because we know that there are stark differences between the principle based design and the design we put in practice.
Stoke’s law is customarily taught when we teach our students the theory of sedimentation. Stokes’ law makes the following assumptions for the behaviour of a particle in a fluid:
- Laminar Flow
- Spherical particles
- Homogeneous (uniform in composition) material
- Smooth surfaces
- That particles do not interfere with each other.
We do not therefore design a sedimentation tank following Stoke’s law. We know its limitations. Instead, we use design criteria such as overflow rate, weir loading rate and retention time to size and arrive at the diameter and depth of the sedimentation tank. But remember that these design criteria make use of the first principles taught in our textbooks as the foundation.
We estimate the likely removal of Suspended Solids (SS) and the Biochemical Oxygen Demand (BOD) based on “performance equations” or “design graphs” where we link performance with overflow rates and retention time. For sewage treatment, these graphs are developed based on actual experience of removals of SS and BOD in practice from data pooled from several installations. So, we tend to believe them and rightly so. (Pity it is that we still do not have in India our “home-grown” national performance equations for designing sewage treatment plants)
Graph indicating BOD and SS Removal for Raw Sewage in a Sedimentation Tank as a function of Overflow Rates and Retention Times
The design of the sedimentation tank is however not complete as all we now have are the outline dimensions of the tank based on design overflow rates and retention time.
For the design that we can implement, we need much more details. We need to decide on the type of influent well, hopper slope, type of scraper mechanism (single or double arm, straight or helical), skimmer, positioning of the motor drive (e.g. at the center or at the periphery) etc. These details are with the manufacturer/supplier of the mechanism or the equipment. Based on the outline dimensions, the manufacturer provides these details along with General Assembly drawings and preliminary cost estimates. The cost estimates include mechanical, civil and electrical works with specifications.
Typical Equipment in the Sedimentation Tank
You learn about these essential elements of design only when you reach the stage of implementation or practice and not in the classroom at the university. We however need to make student understand this important extension.
When I worked with Dorr-Oliver, we were shown videos of installation of equipment for the sedimentation tank (Dorr-Oliver Clarifiers). This made us understand both design features and the procedure of installation – including commissioning. This kind of training helped me to understand link between principles and practice.
To increase the efficiency of sedimentation, chemical assist sedimentation is practiced. Designing chemical assist sedimentation tank requires laboratory studies where we attempt to use different chemicals and arrive at an optimum recipe. The recipe is often a concoction of alum, lime and polyelectrolytes. Based on observations made in the laboratory in a settling column, we come up with the dose of chemicals, estimate the generation of chemical sludge (specially to work out “K factor” (resistance) of the scraper arm, volume of the sludge hopper and the desludging schedule). We then guestimate improved efficiency of SS and BOD based on laboratory experience.
This “base design” however needs to be tweaked in the operational phase, as the experiments made on the settling column do not mimic the hydraulics of the real sedimentation tank. With a “reasonable” base design we adjust the recipe of chemicals during operation. The design process thus extends from first principles (e.g. stoichiometry, understanding flocculation and coagulation), to laboratory trials and finally arrives at the operational design. The students at the university do not get exposed to such a process where principles and operational designs are developed as a logical sequence.
There are situations when it is not possible to arrive at the outline dimensions – especially when designs are proprietary. The textbooks we use do not contain the proprietary information. So, if we want to design a lamella separator instead of a circular sedimentation tank, then we need to reach out to the suppliers of the lamella separator, examine the catalogues, talk to the users or clients and gather the operational experience. Sometimes we need to invite the technology provider or equipment supplier to set up a pilot on the site and demonstrate the performance as Proof of Concept (PoC). With the advent of several types of proprietary equipment, we often land up with discussions with commercial vendors to take final decision. We try to open the “black box” as much possible through questions (based on first principles) and by checking the operational experience.
In all above, we often overlook or underplay the strategy part that should ideally precede the design. We limit ourselves to the engineering design. In the strategy, we need to ask questions such as
- Why do we generate so much wastewater in the first place?
- What is the source of SS?
- Do we need a separate unit for removal of SS at all?
- If yes, then do we use sedimentation or flotation?
- If we use sedimentation, then do we adopt a circular tank or a rectangular tank?
- How will we be addressing situations of increased flows or increased SS concentration?
- How do we handle the sludge?
These questions require a discussion and making of scenarios. We need case studies and a dialogue with the “client”, potential vendor and the practioners. If we follow “flip method” of teaching, then strategy should form the core of the class room sessions.
How should we teach then the engineering design to make it interesting as well as effective?
We clearly need to blend strategy with design and operations. The subject needs to cover the process starting from first principles, laboratory work, outline designs, detailed design (focusing on equipment) followed by tweaks needed based on the operational experience. We need to “teach” in a narrative style.
The lectures will need to include case studies, videos and field visits with faculty drawn from both academia and practice. This is a difficult task and needs good planning and coordination.
I would propose that we conduct Winter/Summer schools of say 2 weeks on engineering design for pollution control following a blended approach. This school may do the necessary blending. I used in this post, illustration of design of sedimentation tank – but indeed we need to cover gamut of major unit operations especially those that have emerged in the last decade where the practice experience has still to mature in India. Examples could be membrane based processes and processes using nano-technology and those operations which use advanced biological processes.
These Winter/Summer schools may not limit to university students but also include professionals. If undertaken as a campaign say across 4 locations in the country, we will be able to train 250 students/professionals every year. A couple of years of experience of running such schools will guide us on how to replicate and scale up.
I wanted to speak with my Professor Friend about my point on blending strategy, design and operations. Professor was at a Sewage Treatment Plant that he frequently visits and conducts field research on what he calls as “operational optimization”.
Professor heard me while we walked towards the aeration tank. He seemed to agree but I saw him not listening to me very attentively.
He inhaled a deep breath near the aeration tank (like a sniff) and turned to the Supervisor “Avinash, I think you need to ramp a bit on the sludge recycling pump, the odor tells me that we are falling short on the MLSS (Mixed Liquor Suspended Solids)”. “Yes Sir” said Avinash.
With this instruction, Professor turned to me.
I decided to block the dates of my proposed Winter/Summer school based on the availability and convenience of my Professor Friend.
Let me know if you are interested to join me.
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