54 September 2013 • Water & Wastes Digest

groundwater discharge

automatedeffluentdisposalBy archis ambulkar, stephen N. Zeller

& David Horvat

Innovat ive deep

trench dr ip disposal

system instal lat ion at

Harford Community

Col lege WWTP

Brinjac Eng. Inc. was hired by Harford Community College (HCC), Bel Air, Md., to replace the college’s existing septic sys-

tem with a new centralized wastewater treatment plant (WWTP) involving a recirculating packed media filter (RPMF)/constructed wetlands treat-ment system and an innovative drip irrigation dosed deep trench disposal system for groundwa-ter 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 f low of 51,000 gal per day (gpd)—with 25,500 gpd of average daily f low—of treated wastewater from this new treat-ment 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.

System SelectionPrior to selecting a drip irrigation dosed deep

trench disposal system, various options were eval-uated for groundwater discharge, including a pres-sure dosed deep trench system. The drip irrigation dosed deep trench disposal system was selected for the following reasons:

• Designofdeepbedscoupledwithconven-tional pressure dosing of the beds would require excessive pump and wet-well capac-ity compared with the drip system due to the need to dose at proper pressure and main-tain proper pressure over the entire trench (approximately 96 ft) and all discharge holes.

• Pressuredosingofthismagnitudewouldresult in challenging hydraulics because the site was on a gentle hill with varying slopes.

• HorvatExcavatingrecommendedusingthedrip irrigation system at HCC because of its simpler design; more consistent application of effluent into the trenches; smaller pumps; consistent dose and higher volume discharge;

automatic cleaning of drip tube; fewer valves/regulators for deep trenches; and esti-mated cost savings of more than $100,000.

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 accom-modating peak flows automatically by decreas-ing the rest time between doses. The presence of a flowmeter offers additional operational monitor-ing. Zones may be brought in and out of service at the control panel, with the controller automati-cally adjusting doses to the active zones. Overall, this is one of the largest drip systems installed in a deep trench configuration in Maryland.

Design BasisThe 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 serv-ing 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 tub-ing for dosing each lateral. In the primary area, three different elevations were used for the later-als 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 lat-eral, and the trench depth to a maximum of 12 ft. There have been a total of 54 trenches in the pri-mary 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 sys-tem for primary and reserve areas:

• Dripsystemfeedwetwell(20,000-galcapacity);• Dripsystemcontrolbuilding:hydraulicunit,

disc filters, UV unit, pump control panel and drip valve controls;

• Septictank,backwashtankandUV recirculation pumps;

Drip tubing in trenchDrip supply/return lines with solenoid,

isolation, air/vacuum release valvestypical drip distribution piping with air/vacuum release valves

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56 September 2013 • Water & Wastes Digest

• Sixzonesinprimaryareasandthreezonesin secondary areas;

• Ninetrenchesperzone;• Twotrenchespercell;

• Sixpiezometersinprimaryandthreepiezometers in secondary areas;

• Oneinspectionportpertrench;• Sixremotevalvesinprimaryandthree

remote valves in secondary areas; and• Oneflowmeterformeasuringflowsto

deep trenches.

Design DetailsThe drip disposal system was pro-

vided 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 eleva-tion 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

groundwater discharge

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groundwater discharge

¾-in. dosing supply lines less than 50 ft to mini-mize friction. Zones 1, 4, 6 and 7 have two sub-zones 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 f lexible 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 tub-ing is passed through a 3-in. perforated PVC pipe for protection and to allow the water to f low out. The end of each lat-eral goes into a return manifold, then into a 2-in. return pipe to the hydrau-lic unit. The return lines, used for f lush-ing, are 2-in.-diameter due to the lower f low rates required. The continuous self-f lushing dripper design flushes debris as it is detected and ensures uninterrupted dripper operation. Microbial growth is controlled by routine, automatic for-ward flushing of the network at a velocity greater than 2 ft per second.

The construction of RAM drip tub-ing,manufacturedbyNetafim,isuniquein that the internal diaphragm and laby-rinth 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

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60 September 2013 • Water & Wastes Digest

groundwater discharge

economically over large areas during controlled periods of time without saturating the surround-ing soil. The emitters have orifices that do not allow roots to enter.

Controls & OperationsEffluent 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 lami-nar 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 control-ler that controls a duplex pump station, alternating the pumps in normal opera-tion, 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 f lowmeter to accu-rately measure f low. The submersible pump delivers eff luent through a UV disinfection system into each filter. The filter backf lushing schedule is triggered at the beginning of each dose cycle. One

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62 September 2013 • Water & Wastes Digest

groundwater discharge

filter valve closes, blocking the f low of unfil-tered eff luent to that filter. After a short delay, the other f lushing valve opens, backf lushing the unused filter. The accumulated impurities

discharge back into the pretreatment unit. The closing and opening procedure of the filter and backf lush valves causes a change of f low within the unit to provide filtered water from one filter

to backf lush the other filter. The backf lush pro-cedure lasts approximately 15 seconds, and then the backf lushing valve closes. Only after the first filter has completed its backf lushing cycle will

the second filter begin its cycle of back-f lushing 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.OperatorsatNCCverifywaterdepth in piezometer (if any) and inspec-tion ports weekly to monitor the disposal beds. Overall, this fully automated system was easy to install and provided a sim-pler solution for the Harford Community College staff and management. WWD

Note: The authors wish to acknowledge Craig Williams, MDE onsite project man-ager, who was instrumental in moving this technology forward in approval for Harford Community College, for this application.

archis ambulkar is environmental engineer for Brinjac eng. stephen N. Zeller is project manager for Brinjac eng. David Horvat is project manager for Horvat excavating. ambulkar can be reach at aambulkar@brinjac.com. Zeller can be reached at szeller@brinjac.com. Horvat can be reached at dmhorvat@comcast.net.

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