Driving Down P

A research project helps establish the cost-benefit picture for chemically removing phosphorus in lagoon treatment systems
Driving Down P
FIGURE 1 – With phosphorus treatment by addition of alum, the cost goes up exponentially as the effluent phosphorus is driven lower.

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State regulatory agencies are ratcheting down nutrient limits for wastewater treatment plants, notably limits on phosphorus. Reaching extremely low phosphorus limits is especially challenging for lagoon systems.

To help demonstrate the levels to which phosphorus can be removed cost-effectively, Pat Morrow, P.E., of MSA Professional Services, headquartered in Baraboo, Wis., conducted a pilot study of treatment by alum addition at the O’Dell’s Bay (Wis.) Wastewater Treatment Facility, a 30,000 gpd (average) covered lagoon facility.

Morrow, who is a certified Wisconsin wastewater operator (Grade 2 activated sludge, disinfection, and Special K-Recirculating Sand Filters) talked about his study design and results in an interview with Treatment Plant Operator.

TPO: What was the impetus for this pilot study?

Morrow: In Wisconsin, there are new water-quality-based phosphorus limits, enacted in 2010. We are the contract operators for the O’Dell’s Bay treatment facility, and we designed an upgrade for it in 2007. So we were in a position to pilot-test alum addition to remove phosphorus. We did that with the permission of the client, who paid for the sampling costs.

TPO: How would you describe the objective of this project?

Morrow: Our aim was to measure how low we could cost-effectively go with chemical phosphorus removal. What happens with chemical treatment for phosphorus is that to achieve lower levels, you have to add more and more chemical. It’s not a linear relationship — those last few pounds of phosphorus you take out at very low concentrations become really expensive.

Wisconsin used to have an effluent limit of 1.0 mg/L for phosphorus. This facility’s discharge ends up in Castle Rock Lake, an impoundment of the Wisconsin River that is an impaired waterway. Under the new water-quality-based rules, they are eventually looking at a 0.03 mg/L phosphorus limit.

Our purpose wasn’t to see if the lagoon system could meet that limit. The purpose was to take as much out as we could and document the cost. Wisconsin allows other avenues such as watershed-based approaches, where you remove phosphorus as best you can, then go out into the watershed and offset the remaining contribution.

TPO: Does this approach have applications for different types of lagoons?

Morrow: All lagoons can remove some phosphorus by addition of alum or ferric chloride. It has been done in multiple types of lagoons with a variety of methods.

TPO: What exactly is O’Dell’s Bay facing with regard to phosphorus limits now and in the future?

Morrow: O’Dell’s Bay is a small sanitary district in central Wisconsin where winter flows are about 20,000 gpd and summer flows can be up to 90,000 gpd because a lot of people have second homes in the area.

They are not required to remove phosphorus now, but they will be. When their next WPDES permit is issued, we expect there will be a requirement to do a feasibility study of whether they can remove phosphorus to meet a future limit of 0.03 mg/L. They will not be able to, but the Wisconsin Administrative Code allows for lagoons to receive an economic variance. To qualify, you need to demonstrate economic hardship.

The Department of Natural Resources has clearly stated that the phosphorus variance is not a get-out-of-jail-free card. It appears that, eventually, lagoons will be required to do whatever is economically feasible to remove as much phosphorus as they can. And they can make a big improvement because the initial pounds of phosphorus at higher concentrations can be removed somewhat readily — you can get a lot of bang for the buck as opposed to doing nothing.

TPO: What is the basic nature of the O’Dell’s Bay treatment system?

Morrow: This district has what textbooks would consider a high-performance aerated lagoon system. There are three lagoon cells in series. In the complete mix cell, enough energy is introduced through aeration to maintain a completely mixed reactor. In the partial mix cell, aeration is supplied for remaining BOD degradation and solids stabilization, but not enough for complete mixing.

The final effluent clarifies in the settling cell, which is followed by UV disinfection. There is an insulated floating cover on the lagoons with hatches to access the fine-bubble diffusers for cleaning or maintenance. They also have influent and effluent flow metering.

TPO: For the pilot project, how was the chemical feed system designed?

Morrow: We fed the alum between the complete mix and partial mix ponds. It was a very simple and inexpensive setup. Next to an existing building we installed storage and delivery tanks for the alum. Inside we mounted a chemical feed pump to a wall. We ran a tube out to a manhole between the two lagoon cells. Finally, we created an alum injection apparatus to ensure effective mixing, thus maximizing contact with the wastewater, so that the alum would grab onto as much of the phosphorus as possible. The aluminum ions from the alum react with phosphate to form an insoluble precipitate.

TPO: How was the actual pilot testing conducted?

Morrow: We completed three discrete pilot runs from July 2010 to September 2011 and in each case collected effluent phosphorus samples on a weekly basis. The theoretical dosing for alum treatment is one mole of aluminum per mole of phosphorus to be treated, but lagoons often need elevated aluminum/phosphorus ratios, and the ratio increases with decreased phosphorus concentration. Our goal was to feed between 3.5:1 and 1.5:1 molar ratios and document removal and associated costs.

TPO: What were the results of the three pilot test runs?

Morrow: We measured influent phosphorus at the outlet of the complete mix lagoon, since some of the phosphorus gets taken up by bacteria in that cell. For the first pilot run, our influent phosphorus was 3.6 mg/L, and we averaged an alum dosage of 3.6:1. At that concentration we were able to get the phosphorus down to 0.43 mg/L. The overall chemical cost per pound of phosphorus removed was $10.88.

For the second test run, we averaged a 1.7 molar ratio. Our influent phosphorus was higher at 6.0 mg/L. Effluent phosphorus was 2.05 mg/L, and the overall chemical cost per pound of phosphorus removed was $5.14.

For the third test run, we chose a middle-of-the-road dosage of 2.2:1, shooting to hit an effluent phosphorus level of about 1.0 mg/L. The influent phosphorus was 4.92 mg/L, the effluent level averaged 0.91 mg/L, and the overall chemical cost per pound removed was $6.65.

 

Summary of Pilot Tests

Pilot Test 1 3.6:1(Alum:P Ratio) 3.6 mg/L(Influent P) 0.43 mg/L(Overall P) $10.88(Overall chemical cost/pound of P removed)

Pilot Test 2 1.7:1(Alum:P Ratio) 6.0 mg/L(Influent P) 2.05 mg/L(Overall P) $5.14(Overall chemical cost/pound of P removed)

Pilot Test 3 2.2:1(Alum:P Ratio) 4.92 mg/L(Influent P) 0.91 mg/L(Overall P) $6.65(Overall chemical cost/pound of P removed)

TPO: What does this exercise demonstrate?

Morrow: It shows that with phosphorus treatment by addition of alum, the cost goes up exponentially as you try to take the effluent phosphorus lower (Figure 1). It goes exponential because you’re talking about diminishing returns. As you keep pumping more alum in, you get less and less proportional benefit out. You get more and more cost, but less and less benefit.

It’s fairly easy and relatively inexpensive to get after phosphorus in the range of 5.0 mg/L to 2.0 mg/L. You don’t have to add a ton of chemical to do that. But as you remove more phosphorus, the incremental cost per pound increases dramatically (Figure 2). Incremental costs represent the additional degree of chemical “effort” needed as you remove phosphorus at lower and lower concentration ranges.

If you’re a fisherman, think about a full minnow bucket. It’s pretty easy to scoop those minnows out with a small net. That’s like going from 6.0 mg/L to a 0.9 mg/L phosphorus concentration. Now think of three guys and one big net in a lake with hardly any minnows in it. That’s like going from a 0.9 mg/L to 0.4 mg/L.

TPO: So, where does the real point of diminishing returns kick in?

Morrow: I would say the diminishing returns really start around 0.8 mg/L, but what’s cost-prohibitive to one community may not be to another. A bigger treatment plant with a lot of users to spread the costs over might see things differently than a small facility that serves perhaps 200 residents. Then again, as treatment plant size increases, the total pounds of phosphorus and associated cost of removal increases.

TPO: Does the type of lagoon affect the performance of alum treatment for phosphorus removal?

Morrow: Covered lagoons have a better chance of reaching low levels of phosphorus. That’s because with a covered lagoon you have much lower effluent TSS. The TSS is very algae-dependent, and you have more algae in an uncovered system. There is phosphorus in the algae itself, and phosphorus also clings to solids particles, so if you have high TSS, you will have a hard time getting low phosphorus.



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