Situated alongside the Arkansas river, LAWRF is a medium-size wastewater treatment plant with a design flow of 54 million gallons a day. The combined primary and secondary sludges are anaerobically digested and then dewatered on four belt filter presses that generate about 4,320 dry tons per year. The biosolids are ultimately classified and transported for use on agricultural fields in the area.
A scaled-up problem
“When we first visited LAWRF about two years ago, they were only treating 30 million gallons of domestic wastewater a day – a fraction of the design flow,” explains Tee Mariga, Senior Sales Manager for Advanced Water Treatment at Kemira. “Struvite scale was building up on the presses themselves, leading to frequent downtime for cleaning and heavily reduced throughput. They were also producing more sludge than they could dewater on their four presses.”
The first step was to evaluate the whole process with a full plant survey. “Full plant surveys are standard practice at Kemira,” Tee points out. “We look at the process holistically so we’re not just seeing one particular angle or problem. We look at what’s going on at the pain point, but just as importantly we look at previous and subsequent parts of the process too.” The full plant survey helped not only to identify the struvite issues, where they were, and their cause, it also helped to discover what changes could be made to unlock additional performance improvements to significantly improve dewatering throughput.
The full plant survey helped not only to identify the struvite issues but also discover what changes could be made to unlock additional performance improvements.
Solving a problem – then optimizing the solution
“We conducted a trial to improve the dewatering efficiency,” explains Tee. “A bench-top belt filter press simulator was used to replicate press operating conditions and we looked at whether adding iron would eliminate the struvite by precipitating the phosphorus. This would also have the added benefit of enhancing dewatering performance. Based on the lab test, the decision was made to initiate a field test and inject the iron into the digester sludge line feeding the presses.”
Savings that keep adding up
The results were impressive. The iron addition provided a solution for the struvite scaling problem by treating the sludge feed line, while removing the phosphorus from the struvite also gave a number of other significant benefits. “Once we had found the solution,” says Tee, “we tweaked it to get even more cost savings for LAWRF, reducing staff hours, doubling throughput, and the equipment run time. The added value to the customer was the expertise in the approach orchestrated with the support of a seasoned Kemira expert, Greg Land. In fact, we eliminated 625 maintenance hours per year as there was no longer any need to mechanically remove the struvite deposits, and 1,877 operation staff hours per year from the increased throughput from 70 to 180 gpm.”
As the plant was understaffed the labor cost savings were a significant benefit. “Previously they had needed to pay for around 3,128 hours of overtime,” explains Tee, “doubling throughput our solution allowed them to reduce the number of overtime hours needed down to 1,200.” Of course, the reduction in operation staff hours also meant an equal reduction in equipment run time, increasing equipment longevity and reducing the dewatering-related KWH energy usage by $20,961 a year.
We eliminated 625 maintenance hours per year as there was no longer any need to mechanically remove the struvite deposits, and 1,877 operation staff hours per year from the increased throughput.
A more nutritious cake
“Our solution meant LAWRF could put twice as much sludge through each individual press,” points out Tee. “This meant they could double the throughput with the same polymer demand, with higher cake dryness and lower costs. Dryer cake means less weight to haul too, saving nearly $30,000 a year in transport costs – not to mention the reduction in accompanying emissions.”
And that cake? It has a lot of nutrients in it; a very good fertilizer for growing crops. Now that it contains iron it is even better – the iron binds the nutrients in the cake so it is not as easily eroded, turning it into a slow-release fertilizer. Finally, because the filtrate now contains less solids after separation, it means cleaner filtrate is being discharged into the Arkansas river. A win for LAWRF – eliminating the recycling costs – and a definite win for the local environment.
