Better Recipe

A new chemical treatment process helps a Florida plant produce Class A biosolids at significantly lower energy and overall operating costs.
Better Recipe
The chemical systems automatically inject a mixture of sulfuric acid, sodium chlorite, sodium hydroxide, ferric chloride and sodium nitrite in two processing steps.

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The Haines City (Fla.) Wastewater Treatment Plant has found a way to recycle its biosolids more easily, make a product for beneficial reuse on agricultural fields, and meet new regulations while cutting operating costs and reducing odors.

The 3 mgd (design) activated sludge plant has an average flow of about 1.5 mgd; effluent is sent to the city's reclaimed water system. The plant needed an upgrade to meet new biosolids regulations from the Florida Department of Environmental Protection that take effect in January 2013. The new provisions substantially limit land application of Class B biosolids and make some land application of Class A biosolids less attractive.

After reviewing alternatives, Haines City selected the Neutralizer process from BCR Environmental. The system treats waste activated sludge with a two-stage chemical system to produce Class A/EQ (exceptional quality) biosolids suitable for commercial fertilizer.

"We are ahead of the curve as far as compliance issues," notes Nathan Silveira, licensed operator and interim pretreatment coordinator at the plant, which serves the city's 20,000 residents. The process allowed elimination of three digesters, each with a pair of 125 hp motors, cutting electrical costs substantially. "We had a lot of odors because of the digesters and that's something that's been eliminated," Silveira adds.

Fast payback

The Neutralizer system was installed in a new 5,000 square foot building on the site of the plant's unused drying beds. The plant expects to save $180,000 in operating costs per year and another $100,000 in energy costs; BCR Environmental says the system uses 5 percent of the energy used by a single digester.

"In the first three weeks, we saw a $6,000 savings in electricity," says Silveira. In the first full month of operation, the savings was documented at about $10,000 and has been as high as $12,000 per month.

The DEP provided a loan for the $4 million project at 2.6 percent interest. Payback is expected in five years. Along with the savings, the investment prevented the expenditure of $2.7 million to rehabilitate the digesters and sludge thickening equipment.

New treatment

After less than a year of construction, the Neutralizer unit was put online in March 2012, and it was fully commissioned in late May. It replaced the old system, which used three aerobic digesters and two drum thickeners to create Class B biosolids.

The plant now processes about 14,400 pounds of biosolids per week in three batches. "When biosolids went to the drum thickeners, we really didn't know how much we were sending," says Silveira. "We were doing 40,000 gallons a day, but our return activated sludge was loose one day and thick another, so we didn't know how much solids we were removing. We had a general idea; now we know precisely."

Turbidity results with the Neutralizer show that substantial solids had been going back to the plant from the drum thickeners. "Over the summer, we usually run at 0.4 NTU turbidity, which is pretty low," says Silveira. "Now we do even better than that. We consistently stay at 0.2 NTU or lower. It has made a difference in how the plant runs, and we saw that difference relatively quickly."

After treatment, what is left is a neutralized biosolid with no odor. It is sent to a centrifuge that dewaters to 19 to 24 percent solids, versus 12 percent with the drum thickeners. The dewatered material is automatically transferred to a trailer with a built-in spreader. "We can fill the process tank in the morning, treat it, dewater it in the afternoon for pickup the next day, and it's off our site," Silveira says.

Previously it took a month to fill a digester, then 40 days for aerobic digestion, which created odors and used electricity the whole time.

Better for operators

While the new system is more technical because of automation, it is better and cleaner for operators. "We have many more parameters to watch on the monitors," says Silveira. "We have cameras so we can see all the different areas. If I see that the trailer is getting full or if something comes up that I have to do, I can pause the system."

There is no material splashing out of the digesters to clean up, and samples are taken from built-in ports. "I don't get messy anymore," Silveira says. "I'm not exposed to the raw sludge nearly as often, and I don't have to deal with the weather or odors."

The Class A biosolids trailer is picked up by a trucking company and land applied on nearby pastures. It can also be used on city properties, such as parks and boulevards. Land spreading of Class B material couldn't have continued under the new regulations. With better treatment, the biosolids can be sold as commercial fertilizer for $250 per 36,000-pound truckload, once the proper permits and licenses are obtained.

Better treatment has also reduced transportation. "We used to haul 45 to 55 loads a month," says Silveira. "With this new process, we're only hauling 12 to 15 times."

That means transportation that used to cost up to $15,000 a month now costs about $6,000. It also cuts carbon dioxide emissions by 180,000 metric tons annually, equivalent to taking 130 cars off the road.

For Silveira, that was just more reason to pursue a new technology that has helped save money and improve wastewater treatment for the community.


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