We hope you were able to attend our last free lagoon webinar: How to Improve the Performance of Failing Lagoons. If not, it’s available to watch at your convenience on our YouTube channel (linked below) or through our Lagooniversity portal.
Webinar attendees posed a bunch of great questions during the Q&A session. Since many are of general interest, we thought we’d share them along with webinar presenter Tom Daugherty’s answers.
Tom Daugherty is Triplepoint’s Western Regional Manager and a wastewater industry veteran with thirty years of administrative and managerial experience, including as president of BlueWater Technologies and as National Sales Manager for s::can. He is a licensed wastewater operator in the state of Washington and holds an MBA in sustainability. Tom’s extensive hands-on experience working with lagoon systems has given him a deep understanding of lagoon operational issues and their solutions.
I know the effect temperature has on lagoon systems. How much do you take temperatures into account when designing the aeration addition? It’s very important. As a default we set winter water temps to 0.5° C and summer water temps to 15° C unless there are site-specific conditions to consider. In very mild climates we will use 4–6° C for winter conditions or defer to engineer’s specification.
Do you recommend types of lagoon transfer valves? Great question. Many of the transfer boxes between cells in older lagoons are a board or plate dam system. Want more water? Add a board to dam it up. Want less? Remove a board. In a new structure look at sluice gates. I don’t have a particular brand to recommend.
Any thoughts on lagoon maintenance for lagoons that are idle during the winter and started up for use at a summer-only facility? If no throughput is available for winter flow I would concern myself with winterization issues such as freeze protecting above-ground idle pipes and the like. From a biological perspective the cell(s) will just go dormant. A bacterial amendment can be used to accelerate a restart. If surface aerators, I would consider taking to shore. If subsurface air, taking to shore is not likely. With subsurface diffused air I would consider a preventative maintenance such as exercising them (turn on blower) once a month for 30 minutes to 2 hours.
Is there any additional nitrogen reduction with the MARS aerators as compared with traditional surface aerators (aspirators)? Somewhat but we do not model TN reduction with air alone, although we find that a well-balanced microbial consortia achieves some natural denitrification. We routinely optimize aeration for nitrification. Denite needs anoxic conditions and a carbon source, something we can absolutely control in our NitrOx reactor. However, we are currently doing some modeling and field trials that involves recycling some nitrified and low DO water back to an anoxic zone, exposing it to soluble BOD, and getting denite polishing. This research portends a less complex solution for facilities who are close on meeting TN land application limits (generally 10 mg/L TN) but need an extra boost. There is still the issue of cold weather suppressing the intended activity.
What is the optimal HRT for each cell? Triplepoint was recently given a blank slate (no land restrictions) to design a lagoon system in California. We designed a two cell system at 12′ water depth at 13.3 days HRT for each cell.
Will catfish eat any Daphnia that are in the polishing pond? My sense is yes but I’ll ping that operator to see if he noted that.
What should you do with the duckweed when skimmed off the lagoon surface? Dry on the dike or further away if possible.
Any suggestions on how best to automate DO measurement in an aerated lagoon (suspend a probe versus pumping sample stream to a probe)? Both have potential issues. I favor suspended if a good auto cleaning system is in place or is added to operator rounds. I would say at least every two weeks for an operator cleaning even though some manufacturer’s claims are longer. The flow cell approach is functional but needs cold weather protection and proximity to the target pond. The flow cell pickup mechanism needs thought to minimize plugging but will still need a screen cleaned periodically. Many have suspended a 5-gallon bucket with multiple ½” holes drilled and the pickup line inside. Might not a bad idea for a suspended probe to be in a bucket also to thwart ragging.
Could you give some more detail on the foaming when you add aeration? We recently also added aeration and was wondering what it is an indication of. Foaming is predominant at a new startup and generally subsides after acclimation in a 4–6 week window. Foaming is mostly off-gassing of various constituents in the water. In municipal water the off-gas is mostly CO2 and nitrogen gas.
Can you share some experience for cold weather operation / lagoon design—how the system operates when there is ice formation on the aerated lagoon? Ice will not form in the Triplepoint aerator diffusion patterns to –50° F. Therefore, natural off-gassing can occur and atmospheric oxygen can interface with the water. As a general statement, biological kinetics slow down with temperature decrease. That is why Triplepoint assumes 0.4° C winter water temp and supplies enough air for that condition which meets all warmer conditions. From a design perspective I would go deeper. Ice formation may be deeper in a long cold winter raising more concern in a 5′ lagoon than a 10′ lagoon.
How can the MARS diffusers help us reduce our ammonia? TPE commonly optimizes aeration for nitrification dependent on effluent concentration goal. We simply add more air above BOD reduction requirements using Metcalf & Eddy’s 4.6 lbs. O2 to remove 1 lb. NH3–N guideline and then allow for head loss across the system (header manifolds, tubing, and aerator device) to deliver net O2. There is still a cold weather concern but many permits have fluctuating effluent values dependent on time of year. However, if your ammonia permit limit is 1.2 mg/L, it’s probably not going to happen with aeration alone. TPE will guarantee 1.2 mg/L with our NitrOx system.
We have, for example, 42 BOD at our effluent outlet, then our wastewater goes into a 500-foot long, 5-foot deep chlorine contact chamber. When it comes out of the CCC we’re getting 48 BOD!? Any thoughts? High probability you have solids buildup in the bottom of the CCC that is releasing. I would scrape around with a poker of some sort and see if you have gunk on the end.
How well does the MARS system do with trash, such as baby wipes? We have surface aerators that clog. What we have seen is some ragging on the tethers that link the aerator to the buoy largely dependent on the type and style of headworks (if any). We have a site in Oregon that installed MARS and did nothing for five years. At the six-year mark they pulled up all the aerators with a floating vessel and pressure washed them. Still going strong and meeting <30 mg/L BOD effluent.
Best methods to remove phosphate with lagoons? I will assume 6 mg/L TP in and 1 mg/L TP out. Tertiary polishing. Cloth disc or drum screen with alum precip or sand filter with iron salts. Biologically there are some EBNR (enhanced biological phosphorus removal) schemes generally reserved for activated sludge plants that can be somewhat replicated in a lagoon but not reliable for 1.0 mg/L TP. There are some more practical CEPT (chemically enhanced primary treatment) schemes that involve dosing ferric chloride in bulk solution at various points in the plant. Ferric binds quite strongly with TP and doesn’t re-release. It actually passes the TCLP test indicating it will not leach. However, there will be solids to reckon with. With tertiary which can reach 1.0 mg/L TP you also have a reject stream to deal with (7–9% Q) that many send back to the lagoon—creating some solids accumulation for eventual resolution.