Farmers and ranchers across Southeast Texas and in states as far away as Florida benefit from the biosolids produced by the Houston wastewater treatment utility.

They’re spreading Class A exceptional quality Hou-Actinite, a 6-3-0 biosolids fertilizer processed and heat-dried at the city’s 69th Street and Almeda Sims wastewater treatment facilities. Some of the material is mixed with chemicals and sold in retail stores for use by landscapers and gardeners.

“People like it,” says Tika Gautam, P.E., supervising engineer of the city’s biosolids management processes. “Customers often ask for more than we have.”

Gautam received the 2021 Ronald B. Sieger Biosolids Management Award from the Water Environment Association of Texas. He earned civil engineering degrees from Tribhuvan University in his native Nepal, and from Lamar University in Texas. In addition to his engineering credential, he holds a Class C Wastewater Operator license and is working to move up to Class B.

VAST OPERATION

Houston Water has one of the nation’s largest wastewater systems. The utility serves 4.2 million residents in an area of more than 680 square miles. Wastewater flows through some 6,300 miles of pipes and 383 lift stations.

The utility maintains and operates 38 wastewater treatment plants with a combined capacity of 250 mgd. The activated sludge plants range from 0.15 to 200 mgd. The goal is to ensure that clean, safe effluent flows from the treatment system to the city’s bayous. Private contractors operate five of the plants under five-year contracts. The utility operates the other 33.

All facilities are permitted by the Texas Commission on Environmental Quality and the U.S. EPA. Houston Water also operates and maintains three wet-weather facilities that collect and store high flows before treatment.

The heart of the system is the 69th Street Wastewater Treatment Plant, permitted to treat up to 200 mgd with a peak flow capacity of 400 mgd. When constructed in 1983, it was the largest treatment facility in EPA Region 6.

The 69th Street facility processes most of the biosolids produced in the Houston Water system. It receives biosolids pumped or hauled in tankers daily from 14 of the smaller plants. The solids are heat-dried to produce about 32,000 dry tons a year of Class A exceptional quality Hou-Actinite slow-nutrient-release fertilizer, also marketed as Sustanite or Cultivate.

The material is sterilized and pelletized, is easy to handle and contains 1% iron and a range of other micronutrients. It is also cheaper than chemical fertilizers. It blends easily, aerates the soil, helps the soil retain water during drought and is not easily washed away.

Currently, almost 13,000 dry tons per year of dewatered cake produced at 10 smaller plants are being landfilled. In the past, these materials were beneficially used as lime stabilized or aerobically digested Class B biosolids.

TREATING THE FLOW

The 69th Street and Almeda Sims plant produce the bulk of the biosolids. At 69th Street (200 mgd design, 80 mgd average), wastewater enters through influent lift stations and passes through multiple mechanical rake bar screens (Infilco Degremont) and aerated grit chambers that include grit pumps (Vaughan), centrifugal blowers (Hoffman & Lamson, by Gardner Denver) and Gritt Mitt classifiers (WesTech Engineering).

The flow is split among eight pure-oxygen-feed nitrification activated sludge process trains, each with a first step covered reactor-clarifier followed by a second-step reactor and clarifier.

Waste activated sludge is pumped to thickeners, and then to dewatering and drying.

Effluent from the second-step clarifiers is polished in 38 newly installed tertiary disc filters (Veolia Water Technologies). The stream is disinfected with liquid chlorine and dechlorinated with sodium bisulfite before discharge to Buffalo Bayou.

At the Almeda Sims plant (20 mgd design, 12 mgd average), wastewater is pumped to multirake mechanical bar screens (Infilco Degremont) and grit removal (Smith & Loveless) before passing to seven open-top aeration basins. Five secondary clarifiers follow.

Secondary effluent flows to chlorine contact basins and then to dechlorination with sodium bisulfite before discharge to Sims Bayou. Waste activated sludge is screened and sent to the thickeners. The thickened solids are stored in digesters/aerated sludge holding tanks, then conditioned, dewatered and heat dried.

BIOSOLIDS RECYCLING

The road to beneficial use of biosolids begins with thickening, digestion and dewatering. The 69th Street plant uses 21 Sharples centrifuges (Alfa Laval) that operate alternately to produce a cake of about 21% solids. The cake is dried in seven Raymond Bartlett Snow flash dryers (Schenck Process) with a total evaporating capacity of 12,000 pounds per hour. The material is stored in 12 silos with a total capacity of 3,300 dry tons.

At the Almeda Sims plant, six Alfa Laval belt presses dewater the biosolids to about 16% solids. Then the material is dried in two rotary drum dryers (Andritz) capable of evaporating 17,000 pounds per hour. The dried pellets are stored in two silos with a combined 600 dry tons capacity.

“The dried biosolids produced at the two plants represent about 70% of Houston’s total biosolids production,” says Gautam. Most of the material loaded from the storage silos into 18-wheeler end-dump trucks for transport to end users. Some of the material fills one-ton bags that are picked up mechanically and hauled to golf courses and other application sites.

Nearly 70 team members are devoted to the biosolids operation. They include specialists from the wastewater treatment plants’ operations, process control, engineering, capital projects, maintenance and other staff, as well as laboratory personnel and senior executives.

GOING TO MARKET

The city partners with Automation Nation of Houston to market and sell its heat-dried fertilizers. “Since 2005, ANI has spent a lot of time and effort in establishing the biosolids beneficial reuse market,” Gautam says.

Marketing the heat-dried product takes many forms. Before the COVID pandemic, the city organized the Houston Garden Show in front of City Hall. There, samples of the heat-dried products, along with flyers describing the production process and the product benefits, were distributed to the public. 

The city also has organized plant visits and tours for local high school and college students, consultants and others. The tours highlight the benefits of biosolids fertilizers.

Houston began its wastewater recycling program in 1921, making it one of the oldest such initiatives in the country. “We are committed to pursuing beneficial biosolids reuse options for our wastewater systems,” says Gautam. “These initiatives protect human health, environmental quality, are cost-effective and provide flexibility.

“In the early days of our operation, when some cities’ wastewater solids were being dumped in the ocean, Houston was shipping heat-dried biosolids pellets by rail to citrus groves in Florida.”

Limited landfill capacities and difficulties getting reuse of Class B biosolids permitted and accepted by the public were the driving factors for the city to invest in facilities to make heat-dried products for marketing and sale.

Those products have been winners for Houston Water, and for the many people who use them.