Magnesium Hydroxide for Collection System Odor, Corrosion, and FOG Control

Introduction

Birch Bay is a protected bay of the east shore of the Salish Sea, a region in the Puget Sound; approximately 100 miles north of Seattle and 35 miles south of Vancouver, BC, Canada.

The Birch Bay Water and Sewer District (BBWSD) provides water and wastewater services to about 7700 connections in northwest Whatcom County in Washington state.

Although unincorporated, most of the district’s water and sewer service areas fall within an Urban Growth Area, which has been one of the most rapidly growing parts of the county in recent years.

In addition to the wastewater treatment plant, the district’s wastewater system consists of about 50 miles of pipe, with 900 manholes and 11 pump stations.

The problem

In order to deal with hydrogen sulfide (H₂S) odor and corrosion, BBWSD has been feeding for a number of years a nitrate-containing product called BIOXIDE, which is designed to encourage the growth of denitrifying bacteria in the wastewater collection system as an alternative to sulfate (SO42-) reducing bacteria that generate H₂S.

The nitrate product was applied to a lift station to treat a force main located about 2 miles from the plant. They were only able to achieve a satisfactory reduction in H₂S with a very high feed rate of the nitrate product, which was not an economical solution. Therefore, over time they settled at an “economical” feed rate that resulted in a consistent ambient H₂S reading of 2-4 ppm, using an H2S sensor installed within the headworks building about 15 feet away from where the influent wastewater entered the facility. This 2-4 ppm ambient H₂S correlated to a 40-80 ppm H₂S concentration when taken right at the influent headworks.

BBWSD was recently notified that they would soon be required to meet a new nitrogen limit in their final effluent. The microbiological process to remove nitrogen, called nitrification, consumes alkalinity, and the influent wastewater to the BBWSD plant was already very low in alkalinity (typically < 75 mg/L). Compounding this problem, they observed more significant losses in pH and alkalinity during the summer months when the nitrifying microorganism activity was highest – but also when the need for odor control was the greatest.

Therefore, they were looking at ways to reduce their nitrogen loads into the plant while also still needing to control collection system odor and corrosion, along with the projected need for a boost in alkalinity and pH at the treatment plant.

The solution

At this time, a representative from BBWSD attended a seminar given by IER discussing the strong alkaline buffering benefits of magnesium hydroxide either for odor and corrosion control in the collection system or for controlling pH in the treatment process.

IER’s ALKA-Mag⁺, containing 60% Mg(OH)₂, was identified as a possible treatment to be fed at the lift station in lieu of BIOXIDE, hoping that the residual alkalinity entering the plant would be sufficient to control nitrification so that caustic soda would also not be required – “killing two birds with one stone”.

They very much liked the nonhazardous nature of ALKA-Mag⁺, as compared to the serious hazards of skin or eye burns that can occur using caustic soda.

_{pH, H2S, and detention time with BIOXIDE and ALKA-Mag⁺}

_{Alkalinity increase}

ALKA-Mag+ is a proprietary, concentrated aqueous suspension of magnesium hydroxide produced from calcined, high purity, natural magnesite. This product is manufactured in Washington state and Iowa using a proprietary blend of dispersants that result in optimum storage stability, feed reliability, and reactivity. ALKA-Mag+ is a safe, non-hazardous means of acid neutralization, coagulation, H2S control, and heavy metals removal in wastewater and process streams.

IER supplies agitated trial storage tanks (from 175 to 1600 gallons) to allow on-site measurement of cost savings and performance – providing customers a clear understanding about how ALKA-Mag+ will improve their overall system before deciding to make a permanent transition. IER supplies ALKA-Mag+ in full tanker or small volume quantities using dedicated, in-house delivery specialists and equipment.

IER provided BBWSD with a 1000 gallon agitated storage tank and metering pump for the trial, with the ALKA-Mag+ being fed into the lift station wet well. The metering pump was activated in unison with the lift station pump.

Unlike traditional magnesium hydroxide suppliers that sell the product in an IBC tote or have it transferred into a poorly agitated storage tank, IER makes sure to supply our customers with a storage tank that is designed for optimal maintenance of the slurry in a well dispersed state. This may be a primary reason that the full benefits of odor, corrosion, and FOG control by magnesium hydroxide in the collection system have never been fully realized until now – as it’s hard to learn the scientific benefits if you are struggling each day simply to feed it!

Prior to the ALKA-Mag+ trial, the typical pH and alkalinity in the treatment plant’s final effluent was 6.9 and 75 mg/L, respectively. Within the first week of treatment, the ambient H2S readings in the headworks building had dropped to “non-detect”, and there was no H2S odor present across the plant.

BBWSD has two force mains that they alternate in use depending on seasonal flows. As they took the one force main off-line they fed a strong dose of ALKA-Mag+ into that line. When that force main was brought back on-line they saw a short blip of about 25 ppm H2S on their ambient meter. Historically they would see readings above 100 ppm for an hour or more.

In addition, the final effluent pH and alkalinity had risen to 7.25 and 105 mg/L, respectively, which was sufficient for controlling the nitrification process. This boost in alkalinity allowed them to turn down their aeration blowers, resulting in significant energy savings.

They very much liked the nonhazardous nature of ALKA-Mag+, as compared to the serious hazards of skin or eye burns that can occur using caustic soda.

_{1000 gallon Agitated Storage Tank for the Trial}

Interestingly, the plant also observed a slight total suspended solids (TSS) increase in the final effluent.

It was assumed that this increase was due to the effect of magnesium hydroxide saponifying and dislodging biomass and FOG in the collection system, resulting in a higher TSS load entering the plant.

To test this theory, the following spring, ALKA-Mag+ was fed to a different lift station where the flow rate of the lift pump could be monitored.

Within the first three weeks of treatment, the flow rate of the pump had increased from 105 gpm to 135 gpm. This cleaning of the pump and line had never been experienced before with a simple chemical treatment, but only by “pigging” the line.

Conclusion

BBWSD was able to use one non-hazardous product, ALKA-Mag⁺, to replace two chemical products (one of which was highly hazardous) to effectively address two performance issues:

• Odor and corrosion control in the collection system
• Nitrification within the treatment plant

In the process they discovered a third benefit of switching from BIOXIDE to ALKA-Mag⁺. By replacing a chemical that fosters biomass growth (BIOXIDE) with one that breaks down insoluble organics (FOG and biomass), the force main became clean – improving the pumping efficiency of the system.

These performance benefits of ALKA-Mag⁺ resulted in cost savings from reduced chemical usage, reduced aeration blower energy usage, and reduced collection system line cleaning costs.

Going into this trial we were hoping to kill two birds with one stone- but were actually able to kill three!!!!