The Los Alisos Water District Water Reclamation Plant is removing raw sewage solids, decayed algae and microorganisms, and algae-laden sludge from its aeration lagoons using a Liquid Waste Technology (LWT) remote controlled lagoon dredging system. The tertiary wastewater treatment plant treats four million gallons per day.
Officials of the Lake Forest, California facility initiated the buildup removal project after study showed that it would increase lagoon storage volume, provide improved detention time, and accommodate increased plant flow.
A long-term system for removal and disposal of lagoon sludge was developed by the engineering firm contracted for the project, Boyle Engineering.
The Los Alisos lagoon system consists of five treatment cells operated in series. Their depths range from 18’ to 30’. Treatment cells 1 and 2 are complete mix aeration lagoons where little settling of solids takes place. Treatment cells 3 and 4 are partial mix lagoons where the majority of suspended solids settle out. Lagoon 3 is 340 by 480 feet, and lagoon 4 is 260 by 540 feet. The LWT dredge has been launched in lagoon 3 and will operate there for about four years before it is moved to lagoon 4.
The LWT Pit Hog™ Electric Remote Control Lagoon Pumper was jointly approved by Boyle Engineering and the general contractor as a result of bids obtained from three dredge manufacturers. Colin Ferguson of Boyle Engineering said that the Pit Hog™ was chosen on the basis of capability and cost savings. LWT auger dredges under consideration would have required special modifications to reach this depth.
The Pit Hog's™ eight-foot auger removes sludge and feeds it to a four-inch, high-capacity submersible cast iron solids handling pump. The pump propels the sludge through a flexible, floating high pressure urethane hose for on-shore collection. An electric motor, coupled to a hydraulic piston pump, provides power to drive the pump auger. The dredge is controlled from shore. Electronic solenoids control the hydraulic systems for pump speeds and depth of the auger.
The dredge is moved along rails installed at each end of the pond. When the dredge reaches the end of a cut, the ladder is raised, the dredge is pulled back to the beginning of the cut, and moved over eight feet to begin the next pass. When the entire pond has been covered in this way, the dredge is moved to the beginning and does the entire pond again with the cutter depth increased.
The dredge is now being controlled by manual remote control, though fully automated dredging is planned, with programmed timing, duration, speed and coverage pattern. Manual operation is more practical at this time due to the high level of solids in the lagoon. One pass of the dredge fills the 12,000 gallon holding tank on shore. Operators pump for two one-hour shifts each morning, and major time savings have been realized because operators do not have to paddle to and from the dredge.
Solids pumped to shore are fed to a filter press for dewatering. A polymer conditioner is added to the sludge to aid dewatering prior to feeding it to the press. Screened compost may also be added as a conditioner, and dewatered sludge cake is composted on-site or hauled away for remote disposal.
The system went on line on January 1, 1994, and has pumped sludge up to 6.2 percent total solids (TS), the highest concentration encountered to date. Solids content pumped at the site averages 5.5 percent TS. The District’s experience is that the dewatering system operates best when pumping less than six percent biosolids, so higher solids content sludge is rarely pumped. The system pumps an average of 25,000 gallons per day. Total tonnage pumped the first year of operation was 620 dry tons, and expected capacity is 75 dry tons per month.
Prior to installation of the dredging system, tests showed that lagoon sludge with solids content up to seven percent TS could be pumped at depths up to 30 feet. Feasibility tests were performed using a variety of systems, including a simple three-inch trash pump on a floating raft, and a 5000-gallon vacuum tank with a weighted intake hose.
In testing, the submersible pumps encountered 7.2 percent TS sludge at a depth of 28 feet. Only 3.3 percent TS was pumped just two feet shallower. The fact that percentage of total solids pumped appeared to increase dramatically with depth indicated that deep penetration would be a primary concern for efficient sludge removal.
An additional criterion for the pumping system was the ability to systematically traverse the lagoon without an onboard operator. Electrically-powered pumping and movement systems were recommended, and an on-shore traversing winch system was to provide both longitudinal and lateral movement. Dredge location and depth were to be controlled using a master remote control panel.
The Los Alisos Water District Water Reclamation Plant expects plant flow to rise to 5.7 million gallons per day by the year 2000. The plant’s maximum capacity is anticipated to be 7.5 million gallons per day. Higher flow rates place greater demands on the lagoon treatment cells, making systematic dredging and pumping increasingly important to plant operation.
'The success of this system is due to the fact that no one else is manufacturing anything quite like it,' said Don Mueller, LWT Vice President.
'We came out with it a few years ago, and have about 12 systems in operation,' said Mueller. 'Several installations are run totally by computer, where the dredge will index the entire lagoon, then move to the beginning and start over, all without an operator present,' he said.
When the dredge is feeding into a pressing plant, a key to the success of the operation is the ability of the dredge to feed a constant percentage of solids. The Pit Hog's™ in-line density and flow meters allow the operator to dial up the desired density, and the dredge will automatically speed up and slow down to maintain that density, said Mueller.
A number of installations are at Superfund sites, notably several nuclear waste cleanup sites where the remote control ability is important.
The Water District received several dozen calls inquiring about the system, said Mueller, a good indicator that the remote control lagoon pumper is filling a niche in the industry.