Septic Systems/Title 5 Glossary

Terms used in the design, installation, and maintenance of septic systems.

Table of Contents


Definitions in this glossary are not official definitions for the purposes of Title 5 regulation and enforcement. See the regulations at 310 CMR 15.000 for official definitions.

Additional Resources


Absorption - The process whereby one substance is physically taken into and included within another substance, as the absorption of water by soil.

Additive - Any solid or liquid material or biological agent marketed primarily for cleaning, treating, degreasing, unclogging, disinfecting, deodorizing or otherwise affecting the performance of any component of an on-site system. MassDEP approves additives for use in Massachusetts, but does not endorse any specific product.

Adsorption - Attachment of a substance to the surface of a solid, such as soil particles or sediment. Many different pollutants can adsorb to soil particles.

Aerobic System - I/A technologies that use aerobic bacteria (bacteria that require oxygen) to treat wastewater and reduce organic material.

Aggregate - Stones of various sizes surrounding soil absorption system. Allows growth of aerobic sewage treatment bacteria.

Alternating Bed Systems - Also known as an alternating leach field. An absorption system designed with a backup absorption field for use while the primary absorption field rests.

Approving Authority - Local Boards of Health are the primary regulatory authorities. MassDEP is involved in certain approvals, such as shared systems at state and federal facilities. MassDEP oversees local implementation of Title 5.

As-Built Plan - Plan that depicts an on-site system (including dimensions, distances from dwellings, roads, surface water, wells) as it was built by a licensed installer.

Additional Resources


Backwash - Wastewater generated from the cleaning of water and wastewater treatment filters.

Betterment - Financial agreement (loan) between a community and homeowner to repair, replace and/or upgrade the on-site system itself, or to hook up to existing sewers.

Biodegradation - Takes place in on-site system soil absorption system (SAS). Organic compounds are broken down into carbon dioxide, water and minerals by the action of microorganisms such as bacteria. 
Blackwater - Wastewater from toilets, urinals, and sinks with garbage disposals.

Buffer - Areas of vegetation left undisturbed or planted between a developed area and a waterbody. Buffer vegetation includes trees, shrubs, bushes and ground cover plants. 

Additional Resources


Centralized Wastewater System - Managed system of collection sewers and a single treatment plant to collect and treat wastewater from an entire service area, in contrast to a system that treats effluent on-site (conventional septic system, innovative/alternative (I/A) system, or cesspool). 

Certificate of Compliance - Certificate issued by the Board of Health to the owner or operator of a system in accordance with 310 CMR 15.021 indicating that an on-site system has been constructed or upgraded, and inspected in compliance with Title 5.  

Cesspool - Pit acting as a settling chamber for solids and leaching for liquids. Cesspools that pose a threat to public health, safety or the environment must be upgraded. See 310 CMR 15.303. 

Cluster System - Wastewater collection and treatment system under some form of common ownership, which collects wastewater from two or more dwellings or buildings and conveys it to a treatment and disposal system that is located nearby. 

Commercial System - Serves a commercial establishment (e.g., retail/office, restaurant or industrial) rather than a residence. Commercial systems serving restaurants and other food preparation facilities must include a grease trap to remove greases and oils before they enter the septic tank (310 CMR 15.230).

Composting Toilet - Technology that uses a biological process to degrade human waste into a humus-like end product.

Conventional Pressure Distribution System - On-site wastewater treatment system consisting of a septic tank and a subsurface soil absorption system that relies on pressure distribution of the effluent.

Conventional Septic System - On-site wastewater treatment consisting of septic tank and a trench or bed subsurface soil absorption system, which relies on gravity to distribute effluent. 

Conveyance Lines - Pipes that connect the various parts of a septic/on-site system (e.g., building sewer, septic tank, distribution box). 

Cover Material - Soils placed on top of a soil absorption system to bring the area to finish grade.

Creeping Failure - Condition in which the biomass of a soil absorption system becomes so intensely developed that no water can flow through it, eventually causing system malfunction. 

Additional Resources


Decentralized System - On-site and/or cluster system used to collect, treat, and disperse or reclaim wastewater from a small community or service area.

Decentralized Wastewater Management - Active oversight of multiple on-site or cluster wastewater treatment systems in a state or local area through regulatory control, licensing, financing assistance and other means, with the intention of protecting the environment and public health, and achieving water quality goals.

Deep Observation Hole - Open pit dug to permit examination of the soils and to obtain data relative to the mean annual high groundwater elevation and depth to impervious materials.

Design Flow - Quantity of sanitary sewage in gallons per day (gpd) for which a system must be designed in accordance with 310 CMR 15.203.

Design Life - Estimated length of time before a system will have to be replaced or rehabilitated. Septic systems have a design life of 20-30 years, given proper siting, construction, and maintenance. 

Disposal System Construction Permit - Written approval by local Board of Health authorizing the construction, upgrade, or expansion of an on-site system.

Distribution Box - Level, watertight structure which receives septic tank effluent and distributes it in substantially equal portions to two or more lines leading to a leaching area.

Distribution Line - Pipe used for dispersion of septic tank effluent into leaching trenches or leaching fields.

Dosing Tank - Watertight structure placed between a septic tank and either a distribution box or a soil absorption system, equipped with a pump designed to discharge septic tank effluent to the soil absorption system and to provide a rest period between such discharges.

Drainfield - See "Soil Absorption System."

Drywell - Pit with open-jointed lining or holes through which stormwater drainage from roofs, basement floors, foundations, or other areas seeps into the surrounding soil.

Additional Resources


Effluent - Sanitary sewage discharged into the environment, whether treated or not.

Effluent Filter - Filter placed at the outlet of the septic tank to trap suspended solids that are not heavy enough or have had insufficient time to sink to the bottom of the tank.


Fill - Clean, uncontaminated, non-indigenous soil placed beneath, above and/or around a soil absorption system.

Flow Diversion Valve - Alternates the flow from the septic tank to one portion of the absorption field while restricting flow to other portions, allowing different parts of the absorption field to be used while other portions rest and recover.


General Use Certification - Approval for innovative/alternative (I/A) technologies that can be used statewide at any site where a conventional Title 5 system can be installed without review and approval of the system design by MassDEP.

Grease Trap - Watertight structure located on a building sewer and before a septic tank in which grease and oils are separated from sewage.

Greenbelt - Area of open space that creates a boundary for development and preserves natural, agricultural, recreational and scenic corridors.

Greywater - Domestic wastewater from all sources except toilets, urinals, and drains equipped with garbage disposals. Sources include washing machines, sinks, showers, bathtubs, and dishwashers.

Groundwater - Fresh or saline water beneath the ground surface contained in spaces between rock and soil particles and bedrock fractures.

Groundwater Elevation - Elevation at which water is observed weeping or flowing from the walls of, or standing in, a deep observation hole.

Additional Resources


Hydraulic Overload - Excessive flows of wastewater that upset the capability of the septic system to properly treat wastewater.


Impervious Material - Material having a percolation rate greater than 60 minutes per inch, including, but not limited to bedrock, peat, loam, silt, clay and organic matter.

Industrial Waste - Any water-carried or liquid waste resulting from any process or industry, manufacture, trade, business, or activity listed in (310 CMR 15.004).

Infiltration Capacity - Maximum rate at which water can infiltrate into soil under a given set of conditions.

Infiltration Rate - Rate at which water penetrates the surface of the soil at any given moment, usually expressed in inches per hour.

Inlet and Outlet Tees - Device on inlet and outlet ports of septic tank. The sanitary tee pipe at the tank inlet slows the incoming rush of water to prevent disturbance to the scum layer and reduce turbulence. The sanitary tee pipe located at the tank's outlet keeps solids, scum and grease from leaving the tank and entering the soil absorption system.

Innovative/Alternative (I/A) System - Any on-site wastewater disposal system or part of one that differs from the design or construction of a conventional or standard septic system. A conventional, or standard, system has a septic tank, distribution box or dosing mechanism, a soil absorption system (SAS), and a reserve area.


Large Capacity Septic System - On-site system having the capacity to serve 20 or more persons per day subject to EPA's Underground Injection Control regulations.

Leach Field - See "Soil Absorption System."

Local Upgrade Approval - Variations of Title 5 that allow system owners to upgrade a nonconforming system to the maximum extent feasible (310 CMR 15.401-405).

Long Term Acceptance Rate (LTAR) - Stable rate of effluent acceptance through the biological mat of a soil absorption system measured in gallons per day per square foot (gpd/sf) (310 CMR 15.242).


Management Model - Program consisting of thirteen elements designed to protect and sustain public health and water quality. This effort is focused on ensuring that on-site and cluster wastewater treatment systems are appropriately managed throughout their life cycle.

Maximum Feasible Compliance - If a failed on-site system cannot be fully upgraded to full compliance with Title 5, the Local Board of Health is authorized to approve an upgrade to bring the system as close to compliance as possible (310 CMR 15.404).

Mounded System - Systems that are constructed in fill, which extends either wholly or partially above natural grade for purpose of complying with the required four or five foot separation to groundwater (310 CMR 15.212).


New Construction - Either a new building or building expansion that causes an increase in design flow above the approved capacity of a system. It does not include any construction that does not include increase in flow.

Nitrogen Reducing Technologies - I/A treatment systems that reduce nitrogen by creating conditions that promote nitrification - the conversion of ammonia (NH3) to nitrate (NO3), followed by denitrification - the conversion of nitrate to inert nitrogen gas (N2). The I/A systems qualifying for a nitrogen reduction credit under Title 5 must be able to reduce nitrogen in wastewater to 19-25 mg/liter from an average influent concentration of 40 mg/liter.

Nitrogen Sensitive Area - An area designated by MassDEP in accordance with 310 CMR 15.215 [310 CMR 15.215], because of its particular sensitivity to the discharge of nitrogen from on-site sewage disposal systems, including drinking water supply areas.

Nonpoint Source Pollution - Pollution that comes from diffuse or multiple sources, such as nutrients and bacteria from a malfunctioning on-site system.


On-site System - Treatment and disposal system for sanitary sewage. On-site systems include conventional septic systems, innovative/alternative (I/A) technologies, and cesspools.

Operating Permit -Renewable and revocable permit to operate and maintain an on-site or cluster treatment system in compliance with specific operational or performance requirements stipulated by the Board of Health.

Owner - Person, who alone or together with another person(s), has legal title to any facility served by a system or control of the facility, including but not limited to any agent, executor, administrator, trustee, lessee or guardian of the estate for the holder of the legal title. 


Peat Filter System - I/A system that filters wastewater through 2-3 feet of peat, after the septic tank but before the soil absorption system (SAS).

Perched Ground Water - Occurring when water, infiltrating the soil from above, reaches an underlying layer of impervious or relatively impervious soil that restricts its downward movement. Perched water can result from heavy rainfall and then disappear in a matter of hours, or the water may remain for months.

Percolation Rate - The rate in which water is absorbed in a percolation hole, expressed in inches per minute.

Percolation Test - A field test for determining the suitability of soil for the subsurface disposal of sewage as described in 310 CMR 15.106.

Permeability - The relative ease (or lack thereof) with which water moves downward through soil. Permeability is measured in inches per hour. The permeability of soil is a critical factor in the suitability/sizing of a leach field for a particular site.

Piloting Approval - Initial step in MassDEP's 3-tier approval process for I/A technologies, requiring field testing and technical evaluation to demonstrate that the technology is likely to provide environmental protection equivalent to a conventional septic system.

Pollutant - A contaminant that adversely alters the physical, chemical, or biological properties of the environment.

Primary Treatment - In on-site systems, the initial treatment (separation of solids from liquids and limited biodegradation of contaminants) that occurs in the septic tank.

Provisional Use Approval - Stage in MassDEP's three-tier approval process for I/A technologies, typically occurring after successful piloting in Massachusetts or satisfactory past performance for two years of general use in one or more other states. Provisional use evaluates the technology's performance under actual field conditions in Massachusetts.


Recirculating Sand Filter (RSF) - A modified version of the old single-pass open sand filter designed to alleviate odor problems associated with dosing partially treated wastewater over a sand filter. A RSF must be used if the system's design flow is 2,000 gpd and is located in a Nitrogen Sensitive Area. See 310 CMR 15.202.

Remedial Use Approval - MassDEP approval of I/A technologies that can improve conditions at existing sites served by a failing, failed, or nonconforming system.

Reserve Area - An area of land with demonstrated capacity for subsurface sewage disposal upon which no permanent structures shall be constructed. This land is intended to be used in case the principal system fails and needs replacing: 310 CMR 15.248.

Retention Time - Length of time effluent is retained in the septic tank. Effluent should be retained in the tank for at least 24 hours to receive adequate primary treatment (i.e., settling out of solids).


Sanitary Sewage - Greywater and blackwater from domestic, commercial and other non-industrial sites.

Scum - A mass of light solids, such as hair, grease, oils and soaps, floating on the surface of the wastewater in a septic tank.

Septage - Material physically removed from any part of an on-site system; contents of septic system.

Septage Hauler - Also known as a system pumper. Licensed by each Board of Health to pump on-site systems and dispose of the contents as part of regular system maintenance.

Septic Tank - A watertight receptacle that receives sewage from a building and provides primary treatment (separation of solids and liquids and partial biodegradation).

Shared System - A system sited and designed to serve more than one facility or more than one dwelling on a single facility and which has been approved in accordance with 310 CMR 15.290 - 15.293. A system serving a condominium unit or units located on the same facility is not a shared system.

Sludge - The heavier solids that separate from wastewater inside the septic tank and sink to the bottom. These solids are subject to continual decomposition activity by bacteria in the tank; however, since breakdown is never complete, solids accumulate and must be periodically removed by pumping.

Soil Absorption System (SAS) - Part of an on-site system: the area of ground and system of subsurface pipes or chambers into which partially treated wastewater from the septic tank or I/A system is discharged for final treatment and absorption by soil. Also called Leach Field, Drainfield or Absorption Field. 

Soil Evaluator - A person approved by MassDEP as capable of evaluating the suitability of soils at a specific site for the use of an on-site subsurface sewage disposal system. The New England Interstate Water Pollution Control Commission (NEIWPCC) administers the training and testing of soil evaluators on behalf of MassDEP.

System Inspector - A person approved by MassDEP as being capable of appropriately assessing the condition of an on-site system. NEIWPCC administers the training and testing of system inspectors on behalf of MassDEP.

Additional Resources


Test Pit - A hole dug in the proposed area for a leach field, to determine soil type, seasonal high water table and depth to bedrock.

Tight Tank - A watertight vessel that has an inlet to receive raw sewage but no outlet. A tight tank is designed and used to collect and store sewage until it is removed for disposal.

Title 5 - On-site sewage disposal systems are governed by Title 5 of the Massachusetts State Environmental Code (310 CMR 15.000). Often erroneously written "Title V," causing confusion with the Federal Clean Air Act.

Trickling Filter - I/A treatment system in which wastewater trickles by gravity through a filter installed between the septic tank and the soil absorption system (SAS). The organic biomass on the filter media absorbs and oxidizes pollutants in the wastewater.


Variance - Variation of Title 5 that occurs when an on-site system cannot meet full regulatory requirements and cannot be approved under a Local Upgrade Approval (310 CMR 15.410-20).


Water Table - The level at which water stands in a shallow well open along its length and penetrating the surficial deposits just deeply enough to encounter standing water in the bottom.

Wellhead Protection Areas - Areas of protection around a public drinking water supply - see in-depth definitions on the Drinking Water website.

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