Harbor Ready Mix of Redwood City, Calif., has installed a prototype reclamation system that not only recycles the sand and gravel from returned concrete, but also the cementitious solids and water. The system's operational merits are potentially applicable to all batch plants that need a cost-effective method to recycle returned concrete without prohibitive investment in new infrastructure.

"Reclaim systems have been around for about 20 years, but I felt the technology only recently met the needs of smaller batch plant operations," notes Harbor Ready Mix General Manager Bob Mann. "Some things applied here were experimental, but the system should fully meet our expectations."

The producer previously disposed of returned concrete at a nearby concrete reclaim center that converts it into road base. Disposing of the returned mix, ranging from five yards up to 30 yards a day, presented a significant cost of doing business, Mann affirms. 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 out to dispose of our returned concrete," Mann says. "An exception might be a large load of five yards in which case we will still use the reclaim center. Otherwise, we will handle it all here. The new system enables us to recycle virtually all of the returned mix and the grey water 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."

Golden State rules

In addition to reducing disposal costs, the new system will help Harbor Ready Mix comply with California's increasingly stringent environmental regulations. About five years ago, the Environmental Protection Agency enacted tougher stormwater regulations, subjecting California producers to a new permitting process and holding them accountable for all water used in their operations. For Harbor Ready Mix, 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 says. "We not only have to account for all of the water the trucks return to the plant but must 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 existing pits and seemed to be the best option."

The system also eliminates the routine cleanout of the square holding ponds common at this and other batch plants. The open pits regularly filled with hardened cementitious 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 nearby Pleasanton. Using three existing basins, which offered 20,000 gallons of water storage, held down Harbor Ready Mix's reclaim system costs.

Innovation + proven methods

Dave Humphrey Enterprises Inc., of Livermore, Calif., was called upon to design the reclaim system that would segregate the sand, gravel, and grey water from unused concrete. Requirements dictated that all recovered concrete ingredients be recycled, including all grey water, typically the most difficult to contain and re-use. Contained within a closed-loop system, the grey water 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. To round out the Harbor Ready Mix prototype system, Humphrey Enterprises applied equipment elements already proven at other batch plants with components derived from unrelated industrial uses.

The recycling process begins when drivers dump their returned concrete into a Rapid model reclaimer, whose trommel screen separates sand from gravel for immediate transfer back to the plant. The remaining cementitious slurry and associated water used to flush and wash the trucks flows by gravity through two 6-in. 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 cementitious 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 grey water is drawn from Pit #1 by an 8-hp Model 2102 ITT Flygt submersible pump and fed back to the plant through a 3-in. line to the batch plant. The recycled grey water is then mixed with either municipal or clarified water from Pit #3 at a controlled ratio to maintain compatibility with specifications of the current batch of mix being blended at the plant. A programmable GE microprocessor, custom software and a specific gravity meter - one commonly used in the brewery industry - keeps the blended water typically balanced at 1.04 specific gravity.

Although Harbor Ready Mix trucks might continue to use the off-site reclamation center for larger returned loads of fresh concrete, the batch plant's recycling system can handle the four to five yards anticipated daily at the plant. The producer finds that it can easily recycle all of the returned mix from the trucks as well as the associated grey water generated from flushing and washing.

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.

Harboring opportunity

Bob Mann and a group of partners started Harbor Ready Mix in 1985 after he had gained 10 years' experience, mostly in sales, in the ready-mix business. Located about 40 miles south of San Francisco, the plant can produce 100 yards an hour, with annual output of 55,000 to 85,000 yards. The plant is one of 15 San Francisco Bay Peninsula ready-mix operations.

The company normally runs eight to 10 conventional 9-yd. trucks and three or four 10-yd. McNeilus Boost-a-Load units. Another eight trucks operate from a sister plant, Coastal Ready Mix, in Castorville.