The National Ground Water Assn. (NGWA) announced that ...
"Reclaim systems have been around for about 20 years, but I felt the technology only recently met the needs of the smaller batch plant operations," reflected Bob Mann, general manager of the batch plant in Redwood City, California. "Some things applied here were experimental but the system should fully meet our expectations."
Harbor Ready Mix previously disposed of returned concrete at a nearby concrete reclaim center that converts it into road base. Disposing of the returned mix, ranging from 5 yards up to 30 yards a day, presented a significant cost of doing business, Mann noted. Reducing the ongoing expense was a primary incentive for having a recycling system at his plant.
"We expect to save most of the $30,000 to $35,000 a year that we previously paid to dispose of our returned concrete," Mann revealed. "An exception might be a large load of five yards in which case we will still use the reclaim center. The system enables us to recycle virtually all of the returned mix and the greywater accumulated from flushing the mixers and washing our trucks. When all things were considered, we projected to recover our investment within four to five years."
In addition to reducing disposal expense, the new system will also help Harbor Ready Mix comply with California’s increasingly stringent environmental regulations. About five years ago, EPA enacted tougher stormwater rules that subject California’s batch plant operators to a permit process and holds them accountable for all water used in the operations. Complying with the stricter regulations, aggravated by increasingly frequent water shortages, further justified the reclaim system.
"Our stormwater rules have gotten a lot tougher," Mann said. "We not only have to account for all of the water the trucks return to the plant, but test any that can flow off the property to ensure it meets acceptable standards. Because of the alkaline nature of our wastewater, I had to either recycle it or triple our holding capacity. The retrofit approach used the 3 existing basins and seemed to be the best option." Offering 20,000 gallons of water storage, the existing basins held down the cost of the new reclaim system.
The system also eliminates the routine cleanout of the square holding ponds common at this and other batch plants. The open pits filled over time with hardened cementious sludge and had to be scooped out with a front loader every six to eight weeks. The material was too alkaline for use as shoreline fill so it was either hauled to the reclaim center, carefully designated spot fills or to a landfill in Pleasanton.
Dave Humphrey Enterprises, Inc., of Livermore, California, was called upon to design the reclaim system that would segregate the sand, gravel, and greywater from unused concrete. Requirements dictated that all recovered concrete ingredients be recycled, including all greywater, typically the most difficult to contain and reuse.
Contained within a closed-loop system, the greywater would ultimately be reintroduced and mixed with municipal water at a controlled rate to achieve an acceptable specific gravity for subsequent batches of concrete. The rest would be filtered and reused at the head end of the reclaimer for flushing and washing the fleet of returning trucks. Dave Humphrey Enterprises, Inc. applied equipment elements already proven at other batch plants with some innovative components derived from unrelated industrial uses.
The recycling process begins when drivers dump their returned concrete into a "rapid reclaimer" whose trommel screen separates sand from gravel for immediate transfer back to the plant. The remaining cementious slurry and associated water used to flush and wash the trucks flows by gravity through two 6-inch diameter pipes into the first of three open pits. Two corrosion-resistant, 15-HP Model 4660 ITT Flygt mixers, commonly used in wastewater treatment plants, are mounted in adjacent corners of Pit #1.
Operating at 580 RPM and running on an alternating cycle, the mixers keep the cementious solids in suspension within the 10,000-gallon pit. Some of the slurry is gravity fed through filters into Pit #2 and subsequently Pit #3 where the clarified water is then pumped back to the head end of the reclaim process.
The majority of the stored greywater is drawn from Pit #1 by an 8-HP Model 2102 ITT Flygt submersible pump and fed back to the plant through a 3-inch line to the 5-yard Ross Rustler 160. The recycled greywater is then mixed with either municipal or clarified water from Pit #3 at a controlled ratio to keep it compatible with the specifications of the current batch of mix being blended at the plant. A programmable GE microprocessor, custom software and a specific gravity meter keeps the blended water typically balanced at 1.04 specific gravity.
In addition to saving disposal charges incurred in the past for returned concrete, the prototype system should help the batch plant comply with increasingly stringent environmental regulations aimed at controlling runoff of tainted stormwater from industrial properties.
Mann and a group of partners started Harbor Ready Mix in 1985. Located about 40 miles south of San Francisco, the plant can produce 100 yards of ready mix an hour — for a total of 55,000 to 85,000 cubic yards a year. This plant is one of 15 plants operating on the San Francisco Bay Peninsula.
The company normally runs eight to ten conventional 9-cubic yard trucks and three or four 10-cubic yard Boost-a-Load units fitted with McNellis Mixers.
For further information contact ITT Flygt Corporation in Trumbull, CT — Phone 203-380-4700.