Got questions? Our expert has answers!
Our Director of Engineering Services answers your wastewater questions
with Bill Hogrewe, Director of Engineering Services, RCAP National Office
After 40 years in the industry, our Director of Engineering Services, Bill Hogrewe, knows wastewater. This month, we’ve asked him some of your wastewater questions.
Question: Has methane capture been scaled down to smaller treatment plants?
Answer: In the wastewater world, almost all the unit operations are aerobic (this reduces odors) and methane generation results from anaerobic processes, so there are only a few places where methane capture would be used. One is an anaerobic pond. These are usually used for industrial wastewaters that are strongly organic. I haven’t seen any attempt to capture the methane coming off of these as they are usually uncovered and very simple. More common is an anaerobic sludge digester. The gas off of these digesters contains a large amount of methane and is always captured. It may be used to generate electricity or to heat the digestion process, or it may just be burned in a flare. Most small plants do not have anaerobic digesters, mainly because they are fairly expensive to build and complex to operate. You don’t start seeing anaerobic digesters until you get to larger plants, maybe 5 mgd (million gallons per day). A good rule of thumb is 100 gal/day/person, so a plant this size would serve around 50,000 people. In my opinion, anaerobic digestion is one of the most complex wastewater treatment unit operations, but it has the potential to recover quite a bit of energy.
Question: Can earthworms be used to treat sludge (vermicomposting or vermiculture)?
Answer: Yes it has been done, and there are lots of papers on how it works, but I haven’t seen any full-scale applications.
Question: What kind of sensors are used in wastewater treatment?
Answer: All the following are used: temperature, flow, Total Dissolved Solids, chlorine. In addition, dissolved oxygen, oxidation-reduction potential, turbidity, pH, and liquid level are also measured. It is interesting to note that most control loops are not closed, i.e. the operator looks at the reading and decides how to adjust the equipment.
Question: Are mobile apps used for process monitoring?
Answer: This is probably getting to be more common. There are a number of PC-based maintenance management programs that help with preventive maintenance reminders.
Question: What’s the thinking about “toilet-to-tap” water re-usage?
Answer: Most drinking water supplies have someone else’s wastewater in them, so we already reuse wastewater. Direct potable reuse is not as common. The City of Aurora, CO has an interesting system that recovers water from the Platte below the Metro wastewater discharge and returns it to Aurora for treatment as drinking water. There are many interesting unit operations employed including: river-bank filtration, aquifer recharge and recovery, chemical softening, UV/H2O2 oxidation, granular filtration, granular activated carbon, and chloramine disinfection. Lots and lots of interesting processes can be used here. Economies of scale play an important role here, making small systems very expensive to build and operate.
Question: Is hazardous waste training required or needed in order to work at a wastewater treatment plant?
Answer: Usually hazardous waste training is not needed at a wastewater plant, but sometimes it is required. It is independent of operator certification, so you can acquire this training at any time.