Septic Tank: Definition & Professional Guide
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A septic tank is a buried, watertight container that receives all wastewater from a building’s plumbing system and provides primary treatment through physical settling and anaerobic biological decomposition before discharging partially treated effluent to a drain field for final soil-based treatment. Residential septic tanks are typically 1,000 to 1,500 gallons in capacity and are constructed from precast concrete, fiberglass, or polyethylene.
Sludge, Scum & Effluent Separation Through Anaerobic Decomposition
When wastewater enters the septic tank through the inlet pipe, it separates into three layers. Heavy solids settle to the bottom, forming a sludge layer that accumulates over time. Lighter materials such as grease, oils, and floating debris rise to the top, forming a scum layer. The middle zone contains relatively clear liquid effluent that flows through the outlet pipe to the drain field.
Anaerobic bacteria naturally present in the sewage decompose organic material in both the sludge and scum layers, reducing their volume over time. However, this decomposition is incomplete — inorganic solids, resistant organic compounds, and excess biological material accumulate and must be removed through periodic pumping. Most septic tanks require pumping every 3 to 5 years, depending on tank size, household size, and water usage patterns.
The septic tank includes an inlet baffle that directs incoming wastewater downward, preventing it from disturbing the scum layer, and an outlet baffle or effluent filter that prevents solids from leaving the tank and entering the drain field. If solids escape to the drain field, they clog the soil pores and cause premature system failure — the most common and expensive septic system problem.
Many older homes were originally built with septic systems before municipal sewer service was extended to their areas. Some properties have since connected to the municipal sewer, while others still rely on their original septic systems or upgraded replacements.
Single-Compartment, Two-Compartment, ATU & Pump Dosing Tank Types
Single-compartment septic tanks have one chamber where all settling, scum separation, and decomposition occur. These are the traditional design and remain common in older installations.
Two-compartment septic tanks divide the tank into two chambers connected by a transfer port. The first compartment receives raw sewage and handles primary settling. The second compartment provides additional clarification before effluent exits to the drain field. This design produces cleaner effluent and is required by many modern codes.
Aerobic treatment units (ATU) add mechanical aeration to the septic tank, introducing oxygen that supports aerobic bacteria. These bacteria decompose organic material more quickly and thoroughly than anaerobic bacteria, producing higher-quality effluent. ATUs are required by many jurisdictions for systems that use restricted drain field types.
Pump tank (dosing tank) is a separate small tank installed after the septic tank that accumulates effluent and periodically pumps a controlled dose to the drain field. Dosing distributes effluent more evenly across the field than continuous gravity flow.
Inspection, Pump-Out Coordination, Baffle Repair & Sewer Connection
Bonded Plumbworks’ septic system services include septic tank inspection, pump-out coordination, baffle repair and replacement, effluent filter installation, and connection of building plumbing to new or replacement septic systems. When septic tank problems cause slow drains, sewage backup, or yard odors, Bonded Plumbworks diagnoses the issue and provides repair or replacement options.
For homeowners considering connecting from septic to municipal sewer — a common transition in developing areas — Bonded Plumbworks handles the full scope of work from septic system abandonment through new sewer lateral installation and building drain connection.
State Health Code, ASTM C1227, IAPMO/ANSI Z1000 Tank Standards
State health departments govern septic system design, sizing, installation, and maintenance. Tank sizing is based on bedroom count — 3-bedroom homes require a minimum 900 gallons in most jurisdictions. Tanks must meet ASTM C1227 (precast concrete) or IAPMO/ANSI Z1000 (fiberglass and polyethylene) manufacturing standards.
Infiltrator Water Technologies, Jensen Precast & Orenco ATU Systems
Infiltrator Water Technologies manufactures the IM Series plastic septic tanks. Jensen Precast produces concrete septic tanks distributed throughout the Southeast. Orenco Systems offers advanced ATU packages. Sim/Tech manufactures effluent filters for septic tank outlets.