Fort Worth’s environmental management system for biosolids provides a roadmap for solving problems and steadily improving processes.
The Fort Worth (Texas) biosolids land application program ran smoothly for more than 15 years. Then along came an odor problem that led to stories on the evening news and complaint calls to the mayor’s office from neighbors of application sites.
It was a challenging problem, but the biosolids team had the recipe for the solution. An environmental management system (EMS) laid out a structured path to help identify the root cause, evaluate potential remedies, and put the best ones into practice.
Today, after the addition of chemical treatment processes, the odor problem has been greatly reduced and the biosolids program is back on track, delivering more than 28,000 dry tons of U.S. EPA Class A material to farm and ranch lands in neighboring counties.
The city’s EMS was first certified by the National Biosolids Partnership in 2005. Last year it passed a third-party reverification audit. “An EMS doesn’t come free,” observes Steven Nutter, biosolids EMS manager in Fort Worth. “There is manpower and paperwork involved. But we have definitely seen value. The EMS provides a structure and a mechanism to continually improve a biosolids program.”
The city’s biosolids are generated at the 166 mgd (design) Village Creek Wastewater Treatment Plant, a regional facility serving Fort Worth and 22 surrounding communities. Primary and waste activated sludges are stabilized in 14 1-million-gallon anaerobic digesters at about three weeks’ retention time. The digested material is pumped through a buried pipeline to a biosolids management facility about 1 mile north.
Contractor Renda Environmental operates the site, dewatering the material on six belt filter presses (Andritz). Renda also handles all aspects of land application, including trucking of the biosolids and all activities related to site permitting and monitoring. In the 2015-16 program year, Fort Worth produced 28,100 dry tons of biosolids, land-applying 80.35 percent and sending 19.65 percent to landfills.
Renda also operates the Village Creek plant’s two 5.2 MW Taurus 60 gas turbines that burn digester gas to generate about 50 percent of the plant’s electricity. A recently installed heat recovery steam generator captures heat from the turbine exhaust to produce steam that powers two centrifugal blowers supplying process air to the aeration basins.
Together, the HRSG and turbine generators meet more than 70 percent of the plant’s overall energy demand.
The biosolids program’s modern history goes back to the early 1990s, when the city spread the digested sludge on drying beds before applying it along highways. “We started having problems with that program,” says Nutter. “One year we had lots of rain, and obviously drying beds don’t work under those conditions. That really backed us up. The need for more drying bed capacity was another issue. So my predecessor and the plant manager at the time looked for alternatives.”
In 1995, the city turned to producing Class A biosolids by way of lime stabilization. “For the longest time, that was a great program,” says Nutter. “It was economical. We were beneficially reusing our biosolids. We got relatively few odor complaints — maybe half a dozen in a bad year. We were going great.”
Then in 2012-13, things changed for the worse, for reasons not fully or immediately clear. “The first thing we noticed was that our biosolids were not dewatering very well,” Nutter says. “The wastewater coming into Village Creek had changed over the past decade, and that meant our biosolids also changed.”
One likely culprit was a significant rise in influent phosphorus, along with a decline in the fiber and ash content. Solids content in the dewatered material dropped from about 16.5 percent on average to 14 percent.
“The difference was like night and day,” Nutter says. “The material was just fundamentally different. We were making mud. When biosolids dewater poorly, that leads to other problems, primarily odor. There are two key issues. One is that the more water you have in biosolids, the more you can get reactivation of the microorganisms. The other issue is polymer. To get the biosolids to dewater, we were having to add a lot more polymer. Then when we added lime to stabilize it, we were creating odors in the field — a kind of a dead fish smell.
“The cationic polymer we use contains a compound called amine. When you add lime to a pH of 12, it attacks the polymer and cuts off that amine. Amine is a precursor to other compounds that create the dead fish smell.” That odor could travel for miles, generating complaints even though the application sites were in sparsely populated areas.
Ben Davis, environmental program manager for Renda Environmental, notes that a state regulatory change in 2007 compounded the problem. Previously, under U.S. EPA rules, when using vector attraction reduction alternative 6, pH-adjusted biosolids could be removed for land application as soon as the pH reached 12.
The new rule from the Texas Commission on Environmental Quality (TCEQ) required 100 percent of the treated biosolids to be raised to at least pH 12 for two hours. The material then had to be maintained above pH 11.5 and kept on site for another 22 hours, in place of performing a regular test to make sure it was meeting the pH requirement. That delay enabled more odor to develop.
Making the fix
The solution to the odor problem had three main components. First is addition of ferric chloride as the biosolids leave the digesters. That has the effect of binding phosphorus into the solids and improving dewatering performance.
Says Davis, “We achieved a 2 percent and sometimes almost 3 percent bump in percent solids. It’s no longer like mud. It’s more of a compost-like material that we can stack on a concrete pad before we load it into trucks.” Besides reducing odor and improving handling, the ferric chloride has cut polymer consumption by 25 percent and has helped prevent struvite buildup in piping.
Next came dosing with chlorine dioxide, a strong oxidizing agent that attacks odor-causing compounds. It is used for odor control in a variety of industries, notably rendering plants. The chlorine dioxide gas, generated on site, is added to a 500,000-gallon tank next to the dewatering building that is fed by the sludge pipeline from Village Creek. An aqueous solution of the gas is metered into the tank to match the incoming flow rate.
“What sold me on that chemical was that if rendering plants use it, then it’s got to work for us,” says Davis. “Sure enough, it did. It’s really, really effective as long as we can get the dosage just right and incorporate it the right way.”
Chlorine dioxide is also a powerful disinfectant, effective against bacteria and enteric viruses. Nutter says, “In the very near future we hope to rely on chlorine dioxide to give us pathogen kill and thus start reducing our dependence on lime. I don’t know if we’ll be able to get away from lime completely, but if we can add lime to achieve pH 10 while adding chlorine dioxide, that’s a one-two punch to give us good pathogen kill and superior odor performance.”
Meanwhile, the city has received approval from the TCEQ to change its vector attraction reduction alternative from lime stabilization to achieving at least 38 percent volatile solids reduction in the anaerobic digesters. That has now been documented through lab testing. As a result, the biosolids no longer have to be stored on site for 24 hours before land application.
Making it all happen
Instrumental in resolving the odor issue was the biosolids program EMS. It lays out the process to follow for solving problems or making improvements that can enhance efficiency, save money and improve performance. “When something comes up, what we call a nonconformance, something that’s causing us issues, we do a corrective action notice,” Nutter says. “It identifies the issue and triggers a root cause analysis: Why are we having this problem? Sometimes it’s a simple answer, and sometimes it’s more complicated.
“Once we do the root cause analysis, the next step is: What are we going to do to fix it? Sometimes that’s easy, sometimes it’s not. Sometimes it’s cost-prohibitive. Then the final step is implementing whatever solution we’ve found. Sometimes that involves money, and other times it can be just a quick process change.”
Nutter’s predecessor Gary Rockers spent about three years creating the EMS with help from a consultant. “In developing an EMS, there are a series of things that need to be codified,” Nutter says. “You’ll have an EMS manual, which is a living document. Underneath that you need goals and targets and standard operating procedures, such as for properly treating, dewatering and applying biosolids. It takes a lot of effort, and a lot of training to make sure all team members are familiar with the procedures.”
Finally, there is an audit process: Every five years an auditor spends several days on site to ensure that the EMS is properly constructed and is being implemented correctly.
Davis emphasizes that the EMS is about more than just dealing with issues: “It also deals with things that aren’t problems — it helps you look at things that could be better, ways to improve the way things already are.”
The EMS came into play as the biosolids site added its sixth belt filter press (Andritz) and fine-tuned its operation earlier this year. The newly designed low-profile press is designed to save space and enable higher throughput of cake higher in solids content. It runs on electricity instead of hydraulics or pneumatics. Renda Environmental and Andritz specialists are working together during the initial setup and operation to help optimize production.
The team will also follow the EMS in a near-future project to potentially increase biosolids dewatering capacity (see sidebar). For Nutter, Davis and colleagues, the EMS is part of a constant effort to improve biosolids product quality and sustain a world-class biosolids management program.
STILL GETTING BETTER
An environmental management system is all about driving continuous improvement. The Fort Worth biosolids program has plans for upgrades in the years ahead.
Most significant is the exploration of increasing dewatering capacity. That dovetails with a project that would add a 5-million-gallon liquid sludge storage tank at the biosolids management facility operated by Renda Environmental.
“Here in Texas, sometimes it can rain for days on end, and we can’t land-apply biosolids when that’s happening,” says Steven Nutter, biosolids EMS manager. “And when it’s raining we don’t want to have stockpiles of dewatered biosolids sitting on site getting rained on. That leads to odors and other problems.”
During rain events, the tank would store liquid sludge pumped to the biosolids site from the Village Creek Wastewater Treatment Plant. After the rainfall, the increased dewatering capacity would enable processing of that stored material along with the flow arriving from Village Creek around the clock. “We would need the additional capacity to get caught up,” Nutter says.
“We’re evaluating all dewatering technologies, belt presses being one of them. We’re also looking at centrifuges, though we realize that centrifuges can have odor issues. We’re examining that very closely.”
In the more distant future, Nutter and his team plan to evaluate biosolids drying: “That may or may not happen. There is lot of capital involved in drying.”