Drive past a farm where City of Muscatine, Iowa, staffers are applying
biosolids and you probably wouldn't know what you were watching.
You'd never see or smell the material. You'd simply see a tractor crawling across the field, apparently doing tillage work — except that it's autumn, the crop is in the bins, and the tractor is attached to a long hose.
City staff members call it an umbilical (or dragline) system: Anaerobically digested material is piped two miles from the city water pollution control plant to a lagoon, and from there it is delivered, again via pipeline, to the injector behind a John Deere tractor. It's a clean, efficient, cost-effective process that helped the plant win the 2011 Biosolids Award for exemplary performance from the Iowa Water Environment Association (IAWEA) in the category for large operating projects.
Muscatine has land-applied biosolids since the mid-1970s. Back then, vacuum-filtered cake was stored in farm fields to be applied with tractors and manure spreaders. Material had to be trucked to the fields, creating a muddy mess in wet weather and clouds of dust in dry weather, according to Roger Kirby, treatment plant director. The process was labor-intensive, and the piles of dark material were unsightly and odorous and attracted insects and rodents.
In 1985, the water pollution control plant added anaerobic digestion, which produced a slurry product. The next year, land became available two miles west of the plant, and the city built two 6.5-million-gallon earthen storage lagoons there. High-density welded polyethylene pipe was installed from the plant to the lagoons so that digested biosolids could be pumped for storage and further treatment.
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The city contracted for 1,200 acres around the lagoons for land application and laid underground pipe from a pump house to hydrants in each field. The city's $600,000 investment in the umbilical system has paid off handsomely, according to Kirby. There are no trucking expenses. Field work requires three people instead of the six it took to run truck-based deliveries. Complaints from neighbors and passers-by are nonexistent.
Of course, the city is fortunate to be able to apply material exclusively on land surrounding the lagoons. It also helps to have an experienced team to handle all aspects of a quality program. The key players are:
Ron Lacina, biosolids team leaderJon Koch, assistant plant director and environmental coordinator, in charge of the industrial pretreatment programBob Jones, plant maintenance managerPatty Fuller-Bloechl, chemist/lab supervisorJim Allen and Steve Brereton, biosolids team mechanics
And finally, there's the material itself. "It's a really good product," says Koch. "We consistently produce biosolids that in most respects qualify as Class A. We're still rated Class B because we don't always have the necessary fecal kill at every point that we test."
The 5.15 mgd (average dry-weather) Muscatine treatment plant uses a conventional activated sludge process. It recently underwent an $18 million upgrade that included three new round primary clarifiers (WesTech), a fine-bubble aeration system with Gardner Denver blowers and Sanitaire (Xylem) diffusers, and two new mesophilic anaerobic digesters designed by Veenstra & Kimm with Gasholder covers and mixers from Olympus Technologies.
The upgrade also included a UV disinfection system (Siemens), because the plant's newest permit now requires disinfection for discharge to the Mississippi River. In the immediate plans are four effluent pumps (three 150 hp and one 100 hp) from Aurora Pump.
"Our topography is such that everything is pumped into this plant and everything is pumped out," says Kirby. "We have no gravity flow. We have 21 lift stations to get the water here and effluent pumps to move it out, although there isn't much head – it's fairly flat from here to the river."
The solids process is simple: Primary sludge and waste activated sludge, the latter thickened to 4 to 5 percent solids by a dissolved air flotation process (Siemens), are pumped to digesters. About 50 percent of the methane from that process fires boilers used to heat the digesters. The balance is flared, although future plans call for burning more methane to heat plant buildings, fuel compressed natural gas vehicles, and possibly to generate electricity.
The digested material is pumped to four 125,000-gallon storage tanks before being piped to the lagoons. The north lagoon stores the biosolids until land application season. "We only apply in the fall at this point," says Lacina. "Farmers don't like running in the spring because of soil compaction."
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During the year, the crew decants liquid from the north lagoon surface into the south lagoon, where it stands for a time to allow the ammonia to flash off before being returned to the treatment plant headworks. The biosolids product applied to land typically contains just over 5 percent solids.
For the field operation, Lacina operates a small dredge pulled by a cable winch. The dredge feeds material through a hose into a wet well, pumping as much as 300,000 gallons per day. From the wet well, the material is pumped underground to field hydrants. A hose is then hooked on to a field hydrant to convey the material to a toolbar injector pulled behind the tractor.
"While I am in the dredge, Jim Allen or Steve Brereton will be running the pumphouse and pulling and testing samples for solids content," says Lacina. "Whichever one of those two is not in the pumphouse will be operating the tractor. It's a very clean process. There's nothing being tracked out on the roads. It's very neighbor-friendly."
The benefits of the umbilical system became all the more clear in 2001, when the owner of 900 of the 1,200 acres around the lagoons declined to renew the biosolids application contract. In 2002, the city bought a used semi-tractor, three used tanker trailers and a new 7,400-gallon slurry tank with an injector toolbar for $133,000 and established a mobile program.
Word of mouth was all it took to sign up six farmers totaling 890 acres with two additional on a waiting list for 500 acres more, all within 17 miles of the water pollution control plant. Biosolids were pumped back to the plant from the lagoons and loaded into the tanker trailers, which delivered the material to the slurry tank on site.
Despite charging farmers $20 per acre during that period to help offset transport expenses, the city incurred significant costs. The landowner who had not renewed then came back in 2010 and signed a new five-year contract.
"Running the umbilical system alone has brought costs down from $130.54 per ton to $72.10 per ton, saving city ratepayers nearly $57,000 in two years," says Kirby. "With new long-term contracts in place, the equipment used for the mobile program could be sold and maintenance costs recouped, adding more than $100,000 in savings."
Good for crops
The biosolids program is based on sound science. Soils are tested in each field for pH and metals. "Since about 2001, we've been testing the soils for metals," Lacina says. "It's not required, but we've had people ask about it, so we started doing that. It's just to see if there has been any increase in metals in the soil. We haven't seen a meaningful increase."
Soil boring sites are marked on a GPS. "We share the sampling information in annual reports and face-to-face meetings with the farmers," says Lacina. "This goes a long way to assure everyone involved that environmental protection of the land and public health is our first priority."
Fuller-Bloechl also runs a standard nutrient test on a composite soil sample. The crew applies biosolids at two dry tons per acre, delivering about 170 pounds of total nitrogen, of which 80 to 100 pounds is available to the crop. In the bargain, farmers typically get 40 to 60 pounds per acre of phosphorus and 12 to 15 pounds of potassium.
The benefits of biosolids to farmers are undeniable. "The farmers on this ground are not buying any commercial nitrogen or phosphorus fertilizer," Lacina says. "They are getting great yields. Farmers don't like to advertise their yields. But a few years ago, one of our farmers had a fertilizer sales rep tell him he needed more nitrogen. That year, he put biosolids on 170 acres of corn ground. I asked him, 'What was your average yield?' He said the farm averaged 195 bushels per acre. I said, 'How much more do you think you could get?'"
Protecting the product
While the material delivers abundant nutrients to the land, Muscatine's industrial pretreatment program helps keep heavy metals content low. The program monitors 12 permitted industries (six of them metal finishers) and 13 septage haulers in and around Muscatine to ensure compliance with strict discharge levels.
The plant's laboratory program is certified by the Iowa Department of Natural Resources, ensuring that sample results are accurate and consistent with required methods. "We test our industries on a quarterly basis for metals," says Fuller-Bloechl. "Some industries of concern are sampled multiple times a week throughout the year to track compliance.
"We also test our influent and effluent, and we do a monthly composite sample for our biosolids coming out of the digesters. That way we can monitor, and if we see any issues, such as metals that are going high, we can look around at the industries in town.
"For metals, we meet the standards for Class A biosolids. Copper is our most prevalent metal, as it is for virtually all cities. The Class B limit for copper is 2,800 mg/kg, and we are usually around 500 to 1,500 mg/kg. The limit for cadmium is 39 mg/kg, and we are less than 7 mg/kg. We're doing really well with our metals."
Koch adds, "The businesses here are very good participants in our pretreatment program. They do very well in reducing the amounts of metals they discharge. People in this role before me did a good job of bringing industries around to the idea that metals need to be removed at the source instead of being sent to our plant."
Looking ahead, the biosolids team was expecting by year's end to renew long-term contracts for the land application sites, whose owners include a retirement community and State Senator Jim Hahn. Successful renewals will lay a firm foundation for many more years of responsible land application of biosolids that benefits Muscatine sewer users and the farming community.
- Biosolids (54)
- Land Application (16)
- Aurora Pump (1)
- Gardner Denver, Inc. (15)
- Olympus Technologies (1)
- Penn Valley Pump Co., Inc. (7)
- Sanitaire - a Xylem Brand (26)
- Siemens Water Technologies Corp. (68)
- WesTech Engineering, Inc. (24)
- Fine-Bubble Aeration System (2)
- Primary Clarifiers (3)
- Blowers (49)
- Diffusers (2)
- Mesophilic Anaerobic Digesters (1)
- Gasholder Covers (1)
- UV Disinfection (76)
- Effluent Pumps (2)