BioCycle May 2010, Vol. 51, No. 5, p. 40
Blending biosolids with ground green waste to make a marketable compost saves this resort community in Puerto Rico $60,000 a year in disposal costs.
Larry Trojak

HOME to more than 6,000 residents, the resort community of Palmas del Mar on the island of Puerto Rico is, in almost every regard, self-contained. With the exception of electricity and fresh water, which it buys from the neighboring town of Humacao, the community operates as a stand-alone entity, providing residents with its own school system, post office, bank, retail shopping, marina, restaurants, sports facilities and golf courses. Lush and green year-round, the nearly 3,000 acre resort generates substantial volumes of green waste from daily trimming, cutting and pruning operations.
According to William Perez, Superintendent for PDM Utilities, disposal of that green waste was once a simple matter. “Palmas del Mar has been in existence for more than 30 years and in the early years we would grind our green waste in a Morbark tub grinder, pile it and give the mulch away to whoever wanted it,” he says. “What wasn’t taken was periodically hauled off to a disposal site. Then, in the mid-1980s, we looked into starting an on-site composting operation.”
PDM Utilities handles sewage, green waste and potable water distribution for Palmas del Mar. Composting would provide the community a secondary use for both the green waste and the wastewater biosolids. And, by creating a product that could be sold back to landscapers and the general public – many of the very customers that were supplying the feedstock – the utility could have a truly closed-loop recycling process.

WASTEWATER TREATMENT EFFICIENCY
Like almost every facet of life at Palmas del Mar, the resort’s wastewater treatment plant (WWTP) has changed dramatically over the years. The 400,000 gallons/day plant, installed when Palmas first opened its gates, has since been replaced with a new facility offering three times the capacity. “The facility uses technology from a German firm called Stahlermatic,” Perez says. “It takes up much less space than a traditional plant and was far less costly, but still offers all the benefits of a larger plant including efficiency, reliability, lack of odor and the right waste product for our composting operation.”
The Stahlermatic process first separates out and treats the biosolids that subsequently are composted. Then, rotating bio-logical contactors (RBC), an efficient fixed-film wastewater treatment technology, treat the wastewater, which is pumped to a polishing tank and used to recharge a pond that irrigates the resort’s golf courses and other landscaping projects.
Green waste from landscapers and other grounds personnel is dropped off at a central location adjacent to the treatment plant, piled, and allowed to sit for several days. Doing so removes excess moisture, which PDM has found enhances the grinding capability. Once it is sufficiently dried, it is run through a Morbark 2600 horizontal grinder, purchased in 2008. The decision to purchase the new machine resulted from seeing a much larger model in use. “That Model 7600 was being operated by the Army Corps of Engineers as part of a hurricane cleanup,” says Perez. “Even though it was much larger than anything we could use, we really liked the way it performed. So when it came time for us to retire the tub, we determined what size machine we needed, and purchased the horizontal grinder.”
The machine, powered by a 250 HP engine, is equipped with the Morbark Integrated Control System, which automatically adjusts feed rates, and monitors pressures and feed wheel position to maximize production and engine efficiency. According to Perez, the unit’s remote operation was also key. “This machine is easy to feed and operate from the cab of the loader, so one person can really do the whole grinding operation. We are a small operation and that helps us a lot.” He adds that troubleshooting can be done remotely via the computerized control system. “Someone at the Morbark plant or one of their facilities monitors the machine’s performance and can identify any problems,” says Perez. “That’s a lot better than having to make someone come all the way out to Puerto Rico.”

COMPOSTING PHASE
After the green waste – which includes palm fronds, coconut shells, grass clippings and tree branches – has been processed, it is set aside for subsequent use. While stockpiled, the ground mulch piles are periodically turned for temperature control. “We use a loader to turn the mulch every three days, mostly just to keep the temperatures down,” he adds. “The heat in the piles can reach 140°F and we like to keep it more around 100°F – turning it every now and then does that. Then, after the mulch has sat for several days, it is ready to be mixed with the biosolids from the plant.”
Mulch and deactivated sludge, which has first been thickened with polymers, are mixed on dry beds near the WWTP. PDM’s crew places a border of mulch around the perimeter of the dry beds. That material serves as a filter, as water from the treated biosolids, which is pumped into the beds from the digesters, drains off, and is recovered for reintroduction into the treatment system. Five to six cubic yards of biosolids and the mulch are combined; the mixture sits for two days, is mixed again and then moved to the composting area where it is placed in windrows, separated and flagged by month.
Once in windrows, the material is turned once every three days which introduces sufficient oxygen to eliminate the need for any forced-aeration process. After about three months, the facility has a marketable compost material. “This has been a really successful program for us,” says Perez. “We figure that we are saving roughly $60,000/year in green waste and biosolids disposal costs alone. And the 200 cubic meters of compost we create every month generates enough revenue to finance the maintenance, upkeep and parts for the entire operation including the grinder. We are constantly looking to improve the process when we can … and to keep this composting operation profitable.”

Larry Trojak of Trojak Communications is based in Ham Lake, Minnesota.