V. Wastewater Assessment - Assessment Approaches/Methodologies
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Assessment Approaches/Methodologies
A number of approaches are suggested below that can be used to determine whether water resources and water quality infrastructure have sufficient capacity to accommodate planned growth and development:
1. Wastewater Treatment Capacity Adequacy
Potential demand and wastewater capacity needs for a planning area should be estimated using the guidance document prepared by MDE, Wastewater Capacity Management Plans (WWCMP), available at MDE's website, www.mde.state.md.us, under the More Publications heading. Page 12 of the WWCMP describes steps used to determine net available capacities for an individual WWTP.
A WWCMP is required to contain information on sewage system capacity and the demand created by existing and projected growth and development. A WWCMP is required by MDE for municipalities operating at 80% design capacity. However, it is recommended that this tool be used for all facilities to help comply with Subsection 9- 512 of the Environment Article that prohibits state and municipal authorities from issuing building permits or approving subdivision plats without demonstrated adequate sewage conveyance and treatment capacity.
The net available capacity estimate can then be converted into estimates of the number or range of households and/or commercial/industrial capacity potentially available to support additional growth in the planning area. To determine the additional number of households that can be served by an existing WWTP, a flow of 250 gallons of wastewater per day per household could be used for planning purposes.
2. Limiting Factors
After determining flow capacity and potential demand, the next step is to consider whether nutrient caps will present a surface water discharge limitation. The more restrictive of the following two nutrient loading caps apply:
* Maryland's Point Source Strategy2 cap to meet Chesapeake Bay water quality standards, and
* Total Maximum Daily Load (TMDL) waste load allocations set to meet local water quality standards.
Proposed treatment plant allocations are most often identified in the Technical Memoranda of the TMDL. However, the final allocation decision is made during the National Pollutant Discharge Elimination System (NPDES) permitting process. Thus, NPDES permit limits are the more authoritative source of information on a plant's nutrient loading limit.
3. Antidegradation Policy
Maryland's antidegradation policy is another factor that needs to be considered as part of a WRE. This policy ensures that water quality continues to support designated uses. Where water quality is better than the minimum requirements specified by the water quality standards, that water quality must be maintained. An antidegradation review is triggered by new or proposed amendments to County Water and Sewerage Plans and discharge permits. The review is required to ensure that water quality remains consistent with antidegradation requirements. The policy states:
“An applicant for proposed amendments to county plans or discharge permits for discharge to Tier II waters that will result in a new, or an increased, permitted annual discharge of pollutants and a potential impact to water quality, shall evaluate alternatives to eliminate or reduce discharges or impacts. If impacts are unavoidable, an applicant shall prepare and document a social and economic justification. The [Maryland] Department [of the Environment] shall determine, through a public process, whether these discharges can be justified."
Planned residential, commercial and industrial growth should not exceed water body assimilative capacity for the planning area. Implementation policies need to be considered to accommodate planned growth. These might include innovative design guidelines, zoning regulations, buffers for sensitive areas and overlay zones. It is recommended that major development be avoided in watersheds draining to high quality water bodies, or Tier II waters.
The regulations identify Tier II waters for which additional protections are required. They are available at www.dsd.state.md.us/comar/26/26.08.02.04-1.htm.
4. Assessing Options
Existing dischargers of any size that want to grow beyond their loading caps will need to consider strategies. These strategies include, but are not limited to, implementation of higher levels of wastewater treatment, wastewater reuse (e.g., spray irrigation), nutrient offsets or trades from other existing dischargers, bubble permits or implementation of nonpoint source reductions. The guidance and regulations for these techniques are currently under preparation by MDE.
Bubble Permits
The bubble permit concept is based on the aggregate annual waste loading of total nitrogen and total phosphorus in pounds per year that is permitted as discharge from all wastewater treatment plants in a given planning area. This approach may provide flexibility in situations where the ability to achieve loading caps varies among WWTPs in the planning area.
Nutrient Trading/Offsets
MDE, with the assistance of stakeholder input, is in the process of developing a policy describing nutrient trading/offsets approaches.
Wastewater Reuse
Spray irrigation can be an important wastewater reuse strategy in planning existing wastewater treatment plant expansion or new discharges under nutrient caps. There are three options for determining spray irrigation site capacity:
Option A provides a rough estimate of site capacity based on the general drainage characteristics of soils in a county without eliminating other limiting factors such as site slope, depths to ground water and bedrock, buffer zone requirements and reserve area. Soil drainage characteristics can be found in the General Soil Map provided in each County Soil Survey. The total site capacity for spray irrigation in a county can be added together after determining the site capacity of each drainage category as indicated by the last column of Table 1 shown in the appendix.
Option B provides a more accurate site capacity estimate by also considering the site limiting factors including site slope, depths to ground water and bedrock, buffer zone and reserve area. Information pertaining to site slope, soil permeability and depth to ground water and bedrock can be found in the County Soil Survey. Please follow the procedures outlined in the appendix to determine the total site capacity for spray irrigating treated wastewater.
Option C provides the most accurate method in determining countywide spray irrigation site capacity because it recommends retaining a soil consultant to conduct a comprehensive hydrogeological study in order to define the spray irrigation rate through soil infiltration tests and the suitable soil boundary through soil test pits or soil borings. Items to be included in the study and approved infiltration testing methods are shown in MDE Guidelines for Land Treatment of Municipal Wastewater available from the MDE web site.
Additional information on the above management approaches to limiting factors is available from MDE's Water Management Administration.
2 Maryland's Point Source Strategy for the Bay established nutrient loading caps for wastewater treatment plants. The Strategy is based on a two-part plan to (1) upgrade Maryland's wastewater treatment plants to state-of-the-art Enhanced Nutrient Removal (ENR) technology and (2) maintain established nutrient loading caps. For additional information regarding the Point Source Strategy please go to www.dnr.state.md.us/bay/tribstrat/implementation_plan/point_source.pdf or contact MDE's Water Quality Infrastructure Program. The nutrient load caps may restrict WWTP ability to expand to accommodate future growth. Other measures would have to be considered in order to accommodate planned development.
