Alum Improves Wastewater Treatment Process

The problem: The wastewater treatment process needs wastewater to flow through the plant, but when struvite crystals build up into a concrete-like substance in the pipes and tanks, they clog the way and stop the process.

The solution: Add alum, a.k.a., aluminum sulfate, to the process. 

Currently, at the Regional Wastewater Treatment Facility, alum is used in the pretreatment phase of the Jeffrey G. Hansen Water Recycling Plant. Every three to four days, tanker trucks deliver 48,000 pounds of alum ($4,255 worth) during the peak summer season. Operators add alum to the pretreatment process to increase the efficiency of the sand filters as they make irrigation water from wastewater.

Alum will also be used in the wastewater treatment process once this $2.15 million alum project is completed in spring 2022. Operators will add alum to the wastewater stream as it enters the treatment plant from the sludge lagoons. Sometimes water is moved from the lagoons back to the treatment plant (during rainy season), and sometimes water is added to the sludge lagoons (during hot summer months). 

This alum will bind up phosphates, causing them to settle to the bottom and making it less likely they will release back into the water.

In the wastewater treatment process, when the liquid stream separates from the solids, it moves on to the aeration tanks where oxygen is pumped in to keep the microorganisms alive while they feed on wastes, breaking them down. If plant operators stop pumping oxygen into the aeration basin, the microorganisms that feed on the phosphates thrive, efficiently removing phosphates from the wastewater stream. 

However, when these microorganisms move on to the digesters, the next step in the wastewater treatment process, they release those phosphates. If there are too many phosphates released back into the wastewater system, they combine with magnesium and ammonium and create struvite, a concrete-like substance that can clog pipes and equipment and is costly to repair.

Adding alum to the wastewater stream provides three benefits. It makes sludge settle easier in the secondary clarifiers so cleaner, less turbid water moves on to the recycled water treatment plant, making that process more efficient. Alum in the wastewater also helps prevent struvite from building up inside the digesters. And finally, because alum improves the efficiencies of the aeration tanks and clarifiers, it will not be necessary for the District to build a fifth secondary clarifier when the community reaches buildout. This will save ratepayers millions of dollars.

Most of the cost of the alum project is for construction of the tanks that hold and disperse the alum into the water from the sludge lagoons before it is returned to the plant for treatment. The alum project engineer is Associate Engineer Kevin Randeni.