Downsizing a School s Wastewater Treatment Facility to a More Cost-Effective On-Site Recirculating Sand Filter Septic System

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Rosanna Richardson
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On-Site Recirculating Sand Filter Septic System New England Water Environment

1 Small Communities Regulatory Issues and Various Ways to Approach Them Downsizing a School s Wastewater Treatment Facility to a More Cost-Effective On-Site Recirculating Sand Filter Septic System New England Water Environment Association 2015 Annual Conference Marriot Copley Place, Boston, MA January 26, 2015

2 Introduction Project Team Project Background Project History Preliminary Design/Permitting Design/Construction Challenges

3 Project Team North Middlesex Regional School District Weston & Sampson Engineers, Inc. R.A. Hammond Construction Corp. Department of Environmental Protection (DEP)

4 Project Background Existing Wastewater Treatment Facility Hawthorne Brook Middle School Approximately 650 people (students/staff) Squannacook Early Childhood Center Approximately 540 people (students/staff) Design Drawings dated April 1988 WWTF designed for approximately 24,000 gallons per day (gpd)

5 Project Location 64 Brookline Street, Townsend, MA

6 Existing Wastewater Treatment Facility Collection system & large septic tank from each school (17,000-gallon and 21,000-gallon) Treatment Building Rotating Biological Contactor (RBC) treatment Secondary clarifier Tertiary filter Dosing chamber 4 submersible pumps Leaching pits/chambers

7 Current Wastewater Flows Average daily discharge from WWTF between 2009 and ,700 gallons per day (gpd) Maximum monthly average discharge during this period 8,000 gallons per day (gpd) No anticipated increase in flow and/or population from schools

8 Project History May 2011 Consultants and School District met with MassDEP to discuss options for WWTF Upgrade WWTF to meet current Nitrogen removal regulations Costly ($1-1.5M) Withdraw Groundwater Discharge Permit and replace WWTF with a Title 5 compliant on-site disposal system Significantly less costly ($500k) and lower O&M costs

9 Wellhead Protection Area (Zone II) Zone II & proximity to Squannacook River

10 Administrative Consent Order On November 22, 2011, MassDEP issued Administrative Consent Order (ACO) Requirement to respond within 90 days, how to bring system into full compliance before December 1, 2013 On-site system must utilize an Innovative/ Alternative (I/A) technology, such as a recirculating sand filter or equivalent, for reduction in effluent Nitrogen December 2011 RFP for Engineering Services February 2012 Weston & Sampson retained

11 Preliminary Design/Permitting Written Plan and Proposed Schedule submitted to MassDEP Variance Request required to replace existing WWTF/GWDP with an Innovative/Alternative (I/A) on-site disposal system

12 Variance Request Average daily discharge (2009 and 2012) 2,700 gallons per day (gpd) Max. monthly average discharge during period 8,000 gallons per day (gpd) Water use records 4,750 gallons per day (gpd) Variance approved for Design Flow of 9,500 gpd

13 Subsurface Investigations Soil testing performed in December 2012 Deep observation holes primarily Sand & Gravel material Percolation test results between 2 and 5 minutes per inch No groundwater or mottling observed Groundwater Monitoring Well within proposed leaching area indicated water at approximately 14 feet below surface

14 Final Design Pressure Distribution system required (>2,000 gpd) Two-compartment tank or tanks in series required (non-residential/>1,000 gpd) Flow Equalization Tank required, due to compressed school flows I/A technology for reduction in effluent Nitrogen MassDEP required the use of Generic Recirculating Sand Filter technology

15 Recirculating Sand Filter Schematic

16 Recirculating Sand Filter Sections Transverse Section Longitudinal Section

17 Proposed System Components Leave existing septic tanks in place (1 st in series) New 10,000-gallon septic tank (2 nd in series) 6,500-gallon flow equalization tank 19,000-gallon recirculation tank 50 x55 recirculating sand filter bed Recirculation splitter valve (4:1) 12,000-gallon pump chamber Two (2) 96 x68 leaching beds

18 Proposed System Layout

19 Decommission Contract included decommission of existing WWTF building (to be used as storage)

20 Construction Permit June MassDEP issued permit for Installation of Recirculating Sand Filter (BRP WP 57) System shall comply with Certification for General Use for the Generic Recirculating Sand Filter (dated September 9, 2008)

21 Permit/Sampling Requirements Certification for General Use Final effluent shall meet the following requirements: 30 mg/l for Carbonaceous Biochemical Oxygen Demand (CBOD5) and Total Suspended Solids (TSS) 25 mg/l for Total Nitrogen (TN) ph between 6.0 and 9.0 Following parameters shall be monitored: ph, effluent CBOD5, TSS, alkalinity and TN

22 Bidding Bid Opening held on June 13, 2013 R.A. Hammond low bidder ($445,000±) Preconstruction Meeting held in July 2013 Construction commenced in October 2013 Based on ACO, system to be in place by December 1, 2013

23 Construction

24 Reconfiguration of Tanks Existing Water Main to WWTF not as shown on Record Drawings Relocation of WM Reconfiguration of proposed tanks

25 Unsuitable Material Unsuitable Material encountered in Leaching Area Compact/tight silty loam material Replaced with Title 5 sand material 670 c.y. of material

26 Recirculating Sand Filter Challenges Sand Filter Enclosure Difficulty achieving watertightness

27 Recirculating Sand Filter Challenges Sand Filter Media Difficulty achieving proper gradation Effective Size: 1 mm to 2 mm. Uniformity coefficient: less than or equal to 3.0. Filter media must be double washed and less than 1 percent (by weight) shall pass a #200 sieve.