Wastewater treatment storage tanks are used for applications ranging from storm run-off storage, sludge digestion and mixing, to sewerage treatment systems. They're used in pulp and paper mills, at mines, oil refineries, steel mills, power plants, textile manufacturing plants, and in the chemical industry. If your layout is set in the modem era, some form of waste wa- ter treatment could be part of just about any heavy industry you might model.
For modeling purposes, what we need to know about the process is that the first step in water treatment is screening out debris, solids and small particles. This requires pumps, strainers and separators, conveniently out of sight in a building for modelers. The l debris is usually taken to landfills.
Next, the wastewater may have chemicals added to precipitate compounds which may be reclaimed for further use or re-processing. This can be done in outdoor or indoor tanks, which tare usually closed to prevent contami- nation. The next step exposes the water 'to air and then filtering it. At this point dissolved gases such as hydrogen sulfide (which smells like rotten eggs) are often released from the water. Oxygen is added, which bubbles up the water and enables it to filter out more organic material; it can also react with some of compounds or organisms in the water. In some processes chlorine is added to kill remaining bacteria, while in other 
processes microbes finish off the bacteria or the impurities. It depends upon what is being treated and the type of factory. Finally, lime is added to soften the water. This results in an exothermic reaction and a pH increase in the water, neutralizing it.
The last stage is filtering the wastewater through beds of activated charcoal, carbon or another material. When the process is complete the water can be recycled by the factory, held in tanks for later use, or released into a waterway. The final processing step is the most visible and usually involves the huge concrete tanks we associate with water treatment.
To make a tank like the one shown here, which is done in HO scale, start used Rust, Grimy Black and a medium gray. Since some spillage shows on the outside of the tank, a containment moat and earth berm should probably surround it. For water in the tank, I mixed a two-part epoxy resin. There are several brands that are suitable although the wastewater is likely to have been somewhat purified by this point, coloring can be added to the resin to tint it. Rusty impurities can be simulated with finely-ground coffee, which makes the water look stagnant and murky.
A catwalk and stairs (painted yellow on my model) were installed. You can raid your scrap box or use commercial products for this. They are made by Plastroct, Walthers and others; etched metal running boards or anti-skid tread can also be used. Catwalks crisscross the huge tanks in many different ways at treatment plants. I finished the scene with a Prieser figure, an oil drum and some hoses made from green wire. I've used solder painted green or red to look like hoses. Since tanks like these are common, finding other details to add is not hard to do.