Coagulation, sedimentation, filtration, adsorption, membranes, activated sludge and digestion as teaching rigs, the wet analytical bench behind them, and the awkward fact that biological processes do not observe a semester timetable.
Most engineering subjects have to invent a case study. Water treatment in Bangladesh does not. Arsenic and iron in groundwater across large parts of the country, salinity moving inland along the coastal belt, dyeing and washing effluent from the garment sector, tannery discharge, and surface water plants trying to hold a treated water standard through a monsoon turbidity spike. A department that connects its rigs to those problems produces graduates who are employable in the effluent treatment sector on the day they leave.
Drinking water treatment and wastewater treatment share physical processes and diverge on everything else: the feed, the objective, the biology and the standards. Nearly every department wants both, and the sensible approach is to buy the shared physical processes first, then take one branch to depth.
The shared spine is coagulation and flocculation, sedimentation, filtration and adsorption. These are the processes that appear in a municipal plant and in an effluent treatment plant alike, they run on a batch of water rather than a live culture, and they can be started and stopped inside a three hour class. That last property is the one that makes them the foundation of the laboratory.
If you buy one item, buy the jar test. A flocculation test unit such as the PEFC is the highest value purchase per taka in the whole laboratory: cheap, indestructible, and it teaches coagulant dose, mixing energy, pH dependence and settling in a single session. It also produces a result students can defend, which matters for report writing.
For the arsenic and iron problem specifically, the teachable process is oxidation followed by coagulation and filtration, which the combination of a jar test unit and a deep bed filter covers well. Do not expect a teaching rig to replicate a field arsenic removal unit. Teach the mechanism on the rig and take the students to a field unit.
A reverse osmosis and ultrafiltration unit such as the ROUC is attractive and is usually bought too early. Membranes teach concentration polarisation, flux decline, recovery and cleaning, all of which are genuinely part of a modern curriculum, particularly for the coastal salinity problem and for pharmaceutical and beverage plant water systems.
The reason to be careful is that membranes are consumable and they are unforgiving. A module fouled by a class that fed it untreated water, or dried out over a long vacation, is a replacement cost the department did not plan for. If you buy a membrane rig, buy a spare module, write a shutdown and preservation procedure, and give one named technician responsibility for it. Otherwise the rig teaches one excellent semester and then becomes a display.
Biological treatment is where a teaching laboratory collides with the academic calendar. An aeration unit (PEAIC), an activated sludge unit (PPFAC), a biofilm process unit (PPBC), an aerobic digester (PDAC), an anaerobic digester (PDANC) and an anaerobic wastewater treatment unit (AWTC) all depend on a living culture. A culture takes weeks to establish, needs feeding daily whether or not there is a class, and dies if the rig is switched off between sessions.
There are only three honest ways to run these, and a department should choose one before ordering rather than after.
Seed sludge is the other practical matter. It has to come from a working plant, which in Bangladesh usually means an industrial effluent treatment plant willing to give you a drum of return sludge. Arrange that relationship before commissioning, and arrange transport, because sludge does not survive a long journey in a hot van. The feed is a related decision: synthetic feed made up to a written recipe gives repeatable results and teaches loading rate cleanly, while real effluent teaches variability and gives far better student reports. Many departments do both, starting synthetic and switching once the culture is stable.
A treatment rig without analysis is a pump moving water. The measurement bench is what turns it into an experiment, and it is routinely under-budgeted because it is unglamorous.
Reagent handling is where a water laboratory becomes a chemical laboratory, with the storage, ventilation and disposal duties that follow. Keep acids and bases in a vented cabinet, keep an eyewash and a shower that has actually been tested, and keep a spill kit where someone can reach it without crossing the spill.
These rigs move a lot of water and produce a lot of noise, smell and floor wetness. Four provisions decide whether the room is usable.
Water and sump. A supply at a known pressure and flow, a sump or storage tank so that rigs recirculate rather than run to drain, and a hose point for cleaning. Recirculation is not only about water cost; it is about not sending a strong synthetic feed into the campus drain.
Drainage with falls. Trapped gullies and a floor laid to fall towards them. Retrofitting drainage into an upper storey laboratory is one of the most expensive changes you can make after handover.
Ventilation. Anaerobic and activated sludge rigs smell, and the smell will travel. Put them in a room with mechanical extraction and put that room away from lecture theatres and offices. Anaerobic digestion also produces a flammable gas mixture, so the vent route and any gas collection has to be designed rather than improvised.
Your own effluent. A university that discharges concentrated synthetic wastewater, spent coagulant, acid and alkali into a municipal sewer or a soakaway is doing at small scale exactly what the course teaches students not to do. Neutralise as a minimum, hold and settle where the load justifies it, and record what leaves the building. It is also the most persuasive teaching point available: the department that measures its own discharge has students who understand why a factory is asked to.