Hatchery Effluent Drum Filter

Fish hatcheries inherently operate with some self cleaning gravity clarifier system functions. The only regular things added to the water is fish food and waste from the reared fish. Phosphates and Nitrogen from feed/waste can contaminate discharge waters if fish are in great enough concentration, waste is not addressed, and/or enough feed fed is not consumed. If the hatchery discharges the byproducts that have settled into the bottom of the rearing tanks/raceways into a controlled discharge location such as a settling pond or treatment center with an equivalent turnover rate per wastewater collecting body of 4 per day - water leaving the facility will have an 85% settle-able solids gravity filtration rate of both the incoming and used water quality leading to a discharge water that is almost as clean as the intake. Facilities that don't route waste to a separate location or with high densities will have to develop treatment solutions, but Dale Hollow is piped with a separate waste line from every tank/raceway.

Dale Hollow NFH has drum filters installed to remove filterable solids from the wastewater. The removed waste is pumped from the effluent building to a lined collection pond large enough to hold approximately 15 years of waste. This test system has proven to have operational limitation that require significant investments of repair skill, maintenance hours, normal usage mechanical degradation and a limited equipment lifespan. 

Core problem and long-term solution:

• Moving/mechanical components in an effluent system result in high maintenance requirements.
• The hatchery has developed a contract ready project scope to change the effluent system from its current design to a gravity clarifier system used universally in wastewater treatment.
• Proposals to modify the hatchery’s effluent are delayed until equipment performance and cost/benefit decline indicate an overhaul is required to provide highest cost efficiency to the USFWS or external funding for the project is secured.
• Even with the regular system complications, hatchery discharge remains within discharge limits thanks to the diligent efforts of hatchery staff.

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Pending system altercation proposal:

1. GENERAL:  Dale Hollow National Fish Hatchery provides aquatic/biological mitigation of Tennessee Valley Authority and U.S. Army Corps of Engineers water development projects in the Southeast and supports USFWS national conservation efforts for threatened/endangered/at-risk species. The hatchery produces 300,000 to 400,000 pounds of fish each year utilizing a feed conversion rate between 1 and 1.25 when corrected for interim fish stocking and attrition. This leaves a biological effluent load of 500,000 pounds of fish waste and excess feed containing regulated nitrogen and phosphates. The removal of these aquaculture byproducts is currently accomplished via a combination of gravity sedimentation and drum mechanical filtration during active physical rearing unit cleaning. Relatively clean water that meets state/federal discharge requirements is passed onto surface waters and the thickened sedimentation is pumped to a confined surface pond.

There are problems with the current effluent system:

• Waste is only diverted to gravity and mechanical separation during active cleaning/draining which allows an unknown percentage of carryover into the hatchery surface water discharge.
• Drum filters require significant routine maintenance that often exceeds station staff manning capacity
• Drum filters are prone to shut-down from clogging, breakdown, or exceeded capability an average of 6 days per month, meaning the station is potentially exceeding discharge permits without the ability to track the amount of waste added to Tennessee surface waters.
• Water flows through the hatchery at a rate of 16,000 gpm with untreated water from the Dale Hollow reservoir through an inlet piping 75’ below the surface

Effluent samples from the facility show consistent reduction in suspended solids as water transfers through the hatchery indicating the gravity settling of sediment in the system exceeds fish feeding and waste production and even reduces the pre-existing solids in the influent supply water. The hatchery is proposing the use of a gravity sedimentation effluent system.

BACKGROUND: Dale Hollow National Fish Hatchery is requesting a replacement effluent system for the facility utilizing technologies employed by other aquaculture programs, the first stage of majority of waste-water treatment plants and a strategic approach customized to the reliability and sustainability of national fish hatcheries. The station will use in-ground cylindrical bottom drain flow-through tanks designed to use slow moving water and gravity sedimentation to thicken and remove solids constantly from the system with minimal moving parts and minimal maintenance and maximal reliability.

1. SCOPE:

General:

• Modify the current hatchery regular discharge to supply a 24” supply header and 16” supply lines for each sedimentation tank.
• Excavate the current hatchery discharge lines and surrounding soil sufficient to safely construct in-ground thickening/waste tanks
• Route 3 phase power supply with an outdoor junction station to supply power to drain pumps, mechanical sludge fan motors, outdoor lighting, VFD controls, spare circuits and local outlets
• Perimeter fencing buried 6 inches in the ground. Chain link with 8’ height connected to the hatchery netting system with support wires and poles.
• Excavation will include tapping into the hatchery discharge and waste lines connecting both lines to the effluent tank supply header with isolation valves gear assist manual diversion valves.
• Each tank requires railed gangplanks spanning the diameter of the tanks.

Cylindrical tanks:

Customization:

• Space estimate to treat hatchery flowrates that meet the design specifications are 8,105 cubic feet of volume necessitating 3 tanks with a cuft of 2701.8 each with 4’ depth and 15’ radius.
• This estimate is based on a cursory review of gravity sedimentation rates for solid particulates specific to a 16,000 gpm fish hatchery producing 400,000 lbs of fish annually.
• These are only preliminary planning numbers used to test the effluent design concept, design for all parameters and capacities are requested as part of the design component of this project.
• Other configuration/system proposals are welcome based on proven designs that meet the goal of low labor autonomous operation to meet facility discharge requirements.

Preparation:

• Soil excavation area near current hatchery clean and waste discharge exposing piping and prepping for tank installation
• Soil and structure structure
  Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.
  
  Learn more about structure requirements for a gravity water effluent system.
• Water diversion and connectivity plan to route water from waste and discharge lines with isolation and diversion capability maintaining the current effluent treatment plant in offline with functional configuration capability.
• Include plans to demo and replace hatchery fencing and netting
• Plan for cement pads for pumps/motors and driving areas connecting the raceways to the current hatchery effluent building parking lot.

Tanks:

• Cement in-ground circular tanks with sloped bases (3 minimum)
• Depth from base to surface less than 4’ to avoid enclosed space limitations for maintenance
• Radius per unit limited to 17’ or less
• Sufficient number of tanks to process 16,000 gpm of water with sufficient primary treatment exposure time to allow collective gravity sedimentation of 1,400 pounds of solid particulate waste per day
• An additional 50% capacity for redundancy, safety, routine maintenance and future expansion ability.
• A smooth, low permeability concrete with minimal cracking potential without the use of seams or other collection points is the desired outcome. The final product must meet or exceed uncoated steel supported fiberglass impregnated concrete.

Piping:

• Supply lines, isolation valves and manually controlled throttle valves (approx. 16”)
• Trough discharge lines (~16”) tied together into a (~24”) discharge line that leads to the creek hatchery discharge.
• A (~6-10”) bottom drainline from the bottom of each tank tied to a (~8-12”) header
• The bottom discharge line tied downhill to above ground sludge pumps with gear-assist quick action dual valve isolation for repairs
• A combined (~8-12”) discharge line to the station sedimentation pond – this line will be combined with the installed effluent system downstream of the check valve with the addition of a manual isolation valve to the line heading to drum filters and both a check and isolation valve on the new tank line (with a lock mechanism added to the tank drainline discharge isolation valve.
• Freeze protection on any above ground piping located between isolation valves (drain line etc)
• Water supply lines for washdown pumps (6-10”), truck fill line (3”), and a frost-free hose spigot for general use.

Equipment:

• 3-phase 220 motors and outdoor electrical supply to operate the shaft driven sludge diversion scraper arm
• Magmeter flowmeters on the clean water outlet line of each tank
• Sludge pumps (as needed for 100% redundancy, approx. 4 units at 70-200 gpm) configured in parallel with VFD and timer local controls
• Quick connect on each sludge pump suction line with a rotating motor mount for manual impeller debris clearing
• 100-150 psi washdown pumps (2 for redundancy) with sufficient flowrate to manually washdown clean the tanks from the walking platform and tank perimeter
• Running lights, power supply lights, overheat and impeller blockage protection for all pumps.

Supplemental work:

• Not all components of the project need have been covered, the design must incorporate all of the project execution and completion components sufficient to supply a construction crew with the full installation scope.
• The engineering design is responsible for contractor support in all phases of construction including overall system performance upon completion.

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Example operational challenge: mechanical failure Oct 2022

All 6 bolts sheared off a drum filter chain drive shaft sprocket coupling. From the oxidation pattern it appears to have been a combination of off-center chain alignment and sequential bolt failure.

Repair summary:

• Troubleshooting, single drum operations, manual effluent controls, equipment (shaft micrometer, thousandths plumb level, surface machining, chain, bolts, key, bearings, motor windings/brushes)
• In-house repairs, total estimate: 50 work hours. Value of labor/parts: $6,850
• Progress: complete