A new system that grinds, washes, compacts, and dewaters bar screenings can dramatically reduce organic content and overall volume, eliminating remote landfill disposal costs and significantly reducing handling needs and odour. The Screenings Washer Monster device, manufactured by JWC Environmental of Costa Mesa, California, will be of interest to wastewater plant managers across North America.
The self-contained unit can process screenings captured by bar screens or other screenings removal devices. Discharge is virtually free of organic (fecal) matter, the exit plug is typically dewatered 40 to 50 per cent and reduced in volume by about 75 to 80 per cent.
Captured solids are diverted from a hopper through a grinder and then passed to an auger that washes and separates soft organics from plastics, paper, and other undesirable elements. Grinding then exposes more surface area for spray water to further clean out unwanted solids.
Organics and wash water pass through the auger’s perforated trough and are returned to the plant’s waste stream. Remaining material is conveyed, compacted, dewatered, and discharged as cake.
A controller accepts the input signal as synchronized with a screen or conveyor, and the augur operates automatically in an on/off cycle. During normal operation, the grinder operates continuously or is sequenced to run only when the controller senses a signal from the screen or conveyor.
Screenings capacities are 150 cu. ft. (2.55 cu. m.)/hour for a 10 HP (7.5 kW) unit, or 25 cu. ft. (0.71 cu. m.)/hr for a 5 HP (3.7 kW) alternative. The unit is also available in a grinder-less version.
“Before the landfill adjacent to our site was closed, all we had to do was haul the screenings there in our own 8 cu. yd. [6.12 cu. m] truck,” recalls Robert Mercaldi, assistant director of the Water Pollution Control Division and manager of a 11.6 million gallons (43.9 million litres) per day plant in Meriden, Connecticut.
The plant, which has won the U.S. EPA Region 1’s Operation and Maintenance Excellence Award twice, installed the screenings washer in May 2001.
After the landfill closure, the plant had to find a company licensed to haul on public roads, certify testing for a list of screenings parameters to be analyzed, rent a 15 cu. yd. (11.47 cu. m) specially-lined dumpster for US$1,200 a month, and pay landfill fees of $40 to 50 a ton. Meanwhile, there were also new tasks involved with handling odorous organics.
Mr. Mercaldi heard that a grinder/shredder already in use in digesters might be used at the front end of the stream in the inlet, but was concerned that the possible re-accumulation of solids in the digester would plug up pumps and heat exchangers. But JWC Environmental had recently developed its new system for this particular situation.
The facility has been able to reduce the weight of its bar screenings from eight tons (7.26 tonnes) per month to 660 lbs. (299.38 kg) per month, and reduce the volume from 15 cu. yd. (11.47 cu. m) per month to 1 cu. yd (0.76 cu. m) per month. In addition to eliminating the cost for the out-of-state dumping and all the work entailed internally, they no longer need the bar screen at the pump station. Screenings formerly removed there can be pumped to the inlet building where the new unit is.
The two remaining bar screens are fine types, with 1-1/2-inch spacing, and automatic raking systems. A single chute receives scrapings from each screen, with effluent water piped in to push them into the screening washer square-funnel hopper. They then move through a grinder and into a wash box where a high-pressure spray cleans the ground up particles and washes out organics. Compression and dewatering follows, with more water squeezed out in the tapered exit chute.
The system is repeated in cycles of 15 to 20 minutes. Back-flow water is used to wash screenings down the chute, and potable water is used for the spray in the wash-box.