Innovative deep trench drip disposal system installation at Harford Community College WWTP
Brinjac Eng. Inc. was hired by Harford Community College (HCC), Bel Air, Md., to replace the college’s existing septic system with a new centralized wastewater treatment plant (WWTP) involving a recirculating packed media filter (RPMF)/constructed wetlands treatment system and an innovative drip irrigation dosed deep trench disposal system for groundwater discharge of the treated sanitary effluent. This is the first application of its kind in Maryland.
HCC submitted an application to the Maryland Department of Environment (MDE) for a permit to discharge a maximum daily flow of 51,000 gal per day (gpd)—with 25,500 gpd of average daily flow—of treated wastewater from this new treatment plant to groundwater. MDE issued discharge limits of 30 mg/L (monthly average) for BOD5, 30 mg/L (monthly average) for suspended solids and 10 mg/L for total nitrogen with a groundwater discharge permit to HCC.
The wastewater treatment process includes primary sedimentation tanks; flow equalization tanks; a recirculating packed media filter, followed by denitrification in a subsurface flow constructed wetland system; and a drip irrigation dosed deep bed trench disposal system. The WWTP has no above-ground components and uses wetlands for polishing the effluent and for denitrification by use of a carbon source (methanol). The proposed design estimates considered an area of approximately 0.5 acres for WWTP construction and approximately 6 acres for the deep trench disposal beds including reserved areas and monitoring wells.
Prior to selecting a drip irrigation dosed deep trench disposal system, various options were evaluated for groundwater discharge, including a pressure dosed deep trench system. The drip irrigation dosed deep trench disposal system was selected for the following reasons:
A dosing regime with drip dispersal utilizes flow equalization and timed dosing, allowing for the spreading of the operationally determined average flow (typically 60%) over a 24-hour period. A system operational interface allows for accommodating peak flows automatically by decreasing the rest time between doses. The presence of a flowmeter offers additional operational monitoring. Zones may be brought in and out of service at the control panel, with the controller automatically adjusting doses to the active zones. Overall, this is one of the largest drip systems installed in a deep trench configuration in Maryland.
The site layout proposed in the Onsite Sewage Disposal System (OSDS) Concept Report, prepared for HCC by another engineer and approved by MDE, was used as the basis for designing the in-ground deep trench system. The proposed deep trench area consisted of: 1) A primary area that would be serving daily flows from the WWTP of up to 40,000 gpd; and 2) a secondary area that would serve as a backup as needed and would provide a capacity of about 20,000 gpd (50% additional capacity) and a reserve area with 150% capacity for future use if needed per MDE regulations. An existing Aberdeen disposal bed on HCC property was proposed to handle the remaining 11,000 gpd of flow from the treatment plant once this bed is remediated to eliminate high nitrogen in the groundwater.
This disposal system involved an inground deep trench absorption area with pressurized drip tubing for dosing each lateral. In the primary area, three different elevations were used for the laterals due to the slope of the site. This was to keep the depth of the trench cover to a minimum of 4 ft, the aggregate to a minimum of 5 ft below the lateral, and the trench depth to a maximum of 12 ft. There have been a total of 54 trenches in the primary area and a total of 28 of the same size in the reserve area (50%).
The following is a brief summary of various components of the deep trench drip disposal system for primary and reserve areas:
The drip disposal system was provided by American Mfg. Its design included six zones or cells in the primary area and three zones for the backup or secondary area. The zones were grouped to minimize piezometers and their resulting monitoring work. Each cell/zone contained a piezometer, extending 8 ft below the trenches, to measure the depth of the groundwater beneath them. When the groundwater is within 4 ft of the bottom, the piezometer would shut off the dosing to its cell.
All cells were constructed of 2- to 3-in. laterals, 96 ft long with 27 drip holes per lateral, or 54 drip holes per cell. The dose per cell was determined by the elevation and length of the lateral. With a 3-ft head, each cell would dispose of 69.12 gal per minute (gpm). For a typical cell composed of two trenches, the total dose would be 355 gal with excess water in the distribution pipe draining back to the dose pump station. There would be 4.17 cycles per day for the entire primary area. This would dispose of the total required peak flow of 40,000 gpd.
The zones have subzones to keep the ¾-in. dosing supply lines less than 50 ft to minimize friction. Zones 1, 4, 6 and 7 have two subzones with eight trenches (laterals) each. Zones 3 and 5 have two subzones with 10 trenches each. Zone 8 has two subzones with subzones on each side of the 2-in. delivery line (10 trenches total). Zone 2 has three subzones (eight trenches total). Zone 9 has three subzones (10 trenches total).
All of the laterals are composed of two runs of 96 ln ft for a total of 192 ln ft of drip tubing per lateral. The distal end of each run has a loop of flexible PVC tubing to connect the second runs. This allows the supply and return line manifolds to be on the same side, along with all of the delivery and return lines (for easier construction). The tubing is passed through a 3-in. perforated PVC pipe for protection and to allow the water to flow out. The end of each lateral goes into a return manifold, then into a 2-in. return pipe to the hydraulic unit. The return lines, used for flushing, are 2-in.-diameter due to the lower flow rates required. The continuous self-flushing dripper design flushes debris as it is detected and ensures uninterrupted dripper operation. Microbial growth is controlled by routine, automatic forward flushing of the network at a velocity greater than 2 ft per second.
The construction of RAM drip tubing, manufactured by Netafim, is unique in that the internal diaphragm and labyrinth provide an exact amount of effluent to be discharged from each of its emitters, which are spaced at 1-ft intervals along the entire length. Each emitter maintains a constant flow of 0.91 gal per hour over pressure ranges from 7 to 70 psi. Because the effluent is distributed at an ultra-low rate, large quantities may be distributed economically over large areas during controlled periods of time without saturating the surrounding soil. The emitters have orifices that do not allow roots to enter.
Controls & Operations
Effluent from wastewater treatment travels into a drip feed duplex pump station containing 60-gpm, 3-hp turbine pumps with check valves to keep water in the delivery lines at all times, and to prevent the pumps from pumping through each other. They are mounted in “coolguides,” or laminar flow collars, which pull the water along the pump to the discharge port located in the center of the pump to keep them cool.
This station has controls similar to a drip system. The heart of the drip control panel is a programmable logic controller that controls a duplex pump station, alternating the pumps in normal operation, reverting to one pump when the first pump fails. Drip controls provide for flow equalization with peak flow management and have been designed to operate on a repeat cycle timer basis. When a float signal would tell the control panel that there is enough water to begin the dose, the timer cycles between a rest time and a run time. Hand-off auto switches allow pump valves to run in a manual mode. The drip controls automatically operate field flushing of the drip tubing and filters.
The control panel is followed by a hydraulic unit, which contains 115-μ disc filters to ensure no solids go into the drip tubing. The hydraulic unit also contains a magnetic flowmeter to accurately measure flow. The submersible pump delivers effluent through a UV disinfection system into each filter. The filter backflushing schedule is triggered at the beginning of each dose cycle. One filter valve closes, blocking the flow of unfiltered effluent to that filter. After a short delay, the other flushing valve opens, backflushing the unused filter. The accumulated impurities discharge back into the pretreatment unit. The closing and opening procedure of the filter and backflush valves causes a change of flow within the unit to provide filtered water from one filter to backflush the other filter. The backflush procedure lasts approximately 15 seconds, and then the backflushing valve closes. Only after the first filter has completed its backflushing cycle will the second filter begin its cycle of backflushing in the same manner as the first.
As of January 2013, the deep trench system is operating with no problems and is providing automated disposal of effluent. Operators at NCC verify water depth in piezometer (if any) and inspection ports weekly to monitor the disposal beds. Overall, this fully automated system was easy to install and provided a simpler solution for the Harford Community College staff and management.
Note: The authors wish to acknowledge Craig Williams, MDE onsite project manager, who was instrumental in moving this technology forward in approval for Harford Community College, for this application.