8.3 Secondary Treatment: Aerated Wastewater Treatment Systems (AWTS) & Media Filters

Source boundary: Use a treatment unit certified to the applicable product standard and follow manufacturer design, commissioning, alarm, disinfection, sampling, and service requirements. Effluent targets and chemicals are consent- and product-specific; never substitute or mix chemicals.

Secondary Treatment: Aerated Wastewater Treatment Systems (AWTS) & Media Filters

While primary septic tanks provide effective gross solids separation, their discharged effluent remains heavily loaded with dissolved organic pollutants, finely dispersed colloidal matter, and pathogenic micro-organisms. In many New Zealand environments—such as properties with poorly draining Category 5 or 6 clays, high seasonal water tables, shallow fractured bedrock, small lot boundaries, or within sensitive lake and groundwater catchments (such as Lake Rotorua, Lake Taupo, and the Hauraki Gulf islands)—primary treated effluent cannot be safely absorbed by the soil. Applying septic effluent in these sensitive settings leads to rapid biological clogging (biomat formation), surface breakouts, foul odors, and severe groundwater contamination.

To overcome these environmental and physical site limitations, certifying drainlayers must specify, install, and commission Secondary Wastewater Treatment Systems. These advanced systems utilize aerobic biological oxidation, mechanical clarification, and chemical or physical disinfection to produce high-clarity, sanitized effluent suitable for shallow subsurface drip irrigation or surface dispersal.

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1. Secondary Treatment Performance Standards

The benchmark standards governing secondary on-site wastewater treatment in New Zealand and Australia are AS/NZS 1547:2012 and AS 1546.3:2017 (On-site domestic wastewater treatment units — Secondary treatment systems).

To be classified as an approved secondary treatment unit, a system must reliably meet or exceed the following effluent quality thresholds over a continuous 26-week standardized performance testing protocol:

Environmental Significance of BOD and TSS Reduction

When primary effluent (BOD5 approx 180 mg/L) discharges into soil, anaerobic bacteria feed on the abundant dissolved organic carbon, producing an impermeable, black gelatinous polysaccharide slime layer known as a biomat. In heavy clays, this biomat completely waterproofs the soil interface, causing the trench to back up and fail. By contrast, secondary effluent (BOD5 <= 20 mg/L, TSS <= 30 mg/L) contains negligible organic substrate, virtually eliminating biomat formation and allowing significantly higher Design Loading Rates (DLR) in compact dispersal areas.

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2. Multi-Chamber Operation of an Aerated Wastewater Treatment System (AWTS)

An Aerated Wastewater Treatment System (AWTS) is essentially a miniaturized municipal activated sludge / fixed-film wastewater treatment plant housed inside a single precast concrete or rotomoulded polyethylene vessel. An AWTS processes wastewater through five sequential treatment chambers:

Chamber 1: Primary Pre-treatment / Settling Chamber

Raw domestic wastewater enters the primary compartment. Exactly like a conventional septic tank, heavy settleable solids sink to the bottom to form a sludge layer, while fats, grease, and scum float to form a surface crust. Anaerobic digestion begins here. This chamber also acts as a hydraulic surge buffer, absorbing momentary peak loads (such as simultaneous bath drainage and laundry cycles). In addition, this chamber receives continuous recirculated biological sludge returned from the downstream clarifier.

Chamber 2: Aeration Chamber (Bioreactor)

Clarified liquid from the primary chamber overflows through a mid-depth baffle into the aeration chamber. This is the biological engine of the AWTS:

• Dissolved Oxygen Injection: An external, continuously operating electric air blower (linear diaphragm or rotary vane compressor, delivering 60 to 120 L/min of air at 15 to 25 kPa) pumps fresh atmospheric air through manifold piping to fine-bubble membrane diffusers mounted on the tank floor. The diffusers release millions of tiny bubbles, maintaining dissolved oxygen (DO) levels between 2.0 and 4.0 mg/L throughout the liquid.
• Biomedia (Fixed Film or Moving Bed): To maintain a high microbial population without washing away during hydraulic surges, the chamber is packed with structured plastic biomedia (corrugated sheets, submerged plastic matrix, or moving-bed bio-rings). This media provides an enormous specific surface area (typically 300 to 600 m2/m3).
• Aerobic Biological Degradation: Complex communities of aerobic heterotrophic bacteria colonize the biomedia as a living zoogloeal biofilm. These aerobic organisms consume dissolved organic compounds, oxidizing them into carbon dioxide, water, and new bacterial cellular mass.

Chamber 3: Secondary Clarification / Settling Chamber

From the aeration chamber, the aerated mixed liquor—containing water and sloughed biological floc—flows into the secondary clarifier. This chamber is quiescent (free of aeration turbulence) and typically features a conical hopper bottom:

• Clarification: The heavy bacterial floc settles to the bottom of the cone under gravity, leaving sparkling clear, polished water at the surface.
• Sludge Return Mechanism (Active Sludge Recycling): Settled biomass on the hopper floor must not be left to turn septic in the clarifier. A low-head airlift pump (driven by a bleed line from the main air blower) or a dedicated submersible timer-controlled pump continuously pulls the settled sludge from the clarifier floor and pumps it back to Chamber 1 (Primary Chamber).

Chamber 4: Disinfection Chamber

Clarified liquid spills over a peripheral serrated V-notch weir into the disinfection chamber to eliminate human enteric pathogens (viruses, bacteria, and protozoa):

• In-Line Erosion Chlorination: Liquid flows through a vertical feeder tube holding solid calcium hypochlorite tablets. As water passes the bottom tablet, chlorine dissolves at a controlled rate, maintaining a free chlorine residual of 0.5 to 2.0 mg/L. A minimum contact retention time of 30 minutes is required before discharge.
• Ultraviolet (UV) Irradiation: Alternatively, water passes through a stainless steel irradiation chamber housing a high-intensity germicidal UV-C lamp (254 nm wavelength) shielded by a high-purity quartz sleeve. UV light penetrates bacterial cell walls and scrambles microbial DNA/RNA, rendering organisms incapable of reproduction. UV requires no chemicals, but requires low turbidity (TSS <= 30 mg/L) so particles do not shield pathogens from the light rays.

Chamber 5: Treated Effluent Pump Chamber

The disinfected secondary effluent collects in the final pump chamber. A heavy-duty submersible multi-stage vortex or centrifugal pump (minimum head capability 25 to 45 m) discharges the treated liquid in controlled doses through a disc filter into the land application network (typically shallow subsurface pressure-compensating drip irrigation lines).

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3. Biological Kinetics: Nitrification & Denitrification

In nutrient-sensitive lake catchments (such as Rotorua Te Arawa Lakes or Lake Taupo), regional councils strictly regulate total nitrogen discharges to prevent eutrophication, toxic cyanobacterial algal blooms, and dissolved oxygen depletion.

Secondary AWTS units achieve biological nitrogen removal through a coupled two-stage microbial pathway:

1. Aerobic Nitrification (Occurs in Chamber 2 - Aeration): Raw sewage contains high concentrations of Total Kjeldahl Nitrogen (TKN), primarily in the form of organic nitrogen and dissolved ammonia (NH3 / NH4+). Under warm, aerobic conditions with DO >= 2.0 mg/L, slow-growing autotrophic bacteria oxidize ammonia:

   • Nitrosomonas bacteria oxidize ammonia to nitrite:
     2NH4+ + 3O2 -> 2NO2- + 4H+ + 2H2O
   • Nitrobacter bacteria oxidize nitrite to nitrate:
     2NO2- + O2 -> 2NO3- This process consumes significant alkalinity, which can lower water pH if natural water is soft.

2. Anoxic Denitrification (Occurs in Chamber 1 - Primary Settling via Sludge Return): The nitrified mixed liquor containing high nitrate (NO3-) is returned to Chamber 1 by the sludge return airlift pump. Chamber 1 is completely anoxic (no dissolved oxygen) but rich in raw soluble carbon (BOD). Facultative heterotrophic bacteria (Pseudomonas and Bacillus) utilize the nitrate molecules as terminal electron acceptors for respiration, stripping oxygen and reducing nitrate into harmless inert nitrogen gas (N2): 2NO3- + 10e- + 12H+ -> N2 (gas) + 6H2O The harmless nitrogen gas bubbles out to atmosphere, reducing total nitrogen in the final effluent by 50% to 75%.

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4. Alternative Secondary Technologies: Textile Filters, Sand Beds & Wetlands

While mechanical AWTS units dominate the suburban market, alternative secondary technologies provide excellent performance, particularly for intermittent holiday home use where continuous aeration is problematic:

Packed Bed Textile Reactors (e.g. Orenco AdvanTex)

• Mechanics: Raw wastewater settles in a two-compartment primary tank fitted with a high-surface effluent filter. Clarified liquid is pumped to an above-ground or in-ground insulated basin containing vertically hung engineered non-woven textile fabric sheets.
• Process: Small submersible pumps dose effluent over the top of the textile sheets via low-pressure distribution manifolds for 15 to 30 seconds every 15 minutes. Liquid trickles as a thin, highly oxygenated film over the porous textile, where an extremely dense microbial biofilm digests BOD and nitrifies ammonia. Over 80% of the drained filtrate recirculates back to the primary tank for denitrification.
• Advantages: Extremely low electrical power consumption (uses less than 1.5 kWh/day, compared to 3 to 5 kWh/day for an AWTS blower); silent operation; highly resilient to fluctuating or seasonal baches/holiday homes.

Intermittent Sand Filters (ISF) & Recirculating Sand Filters (RSF)

• Mechanics: Lined underground beds filled with 600 mm to 900 mm of washed, graded coarse sand (effective grain size D10 = 0.3 to 0.5 mm, uniformity coefficient Uc < 4.0).
• Process: Timed pressure doses apply liquid evenly over the surface via slotted lateral pipes bedded in gravel. As liquid percolates downward, physical filtration, chemical adsorption, and aerobic biofilms renovate the water, achieving exceptional clarity (BOD5 < 5 mg/L, TSS < 5 mg/L).
• Trade Limitation: Requires a substantial land footprint and access to large volumes of certified washed silica sand.

Constructed Subsurface Wetlands (Reed Beds)

• Lined gravel beds (depth 400 to 600 mm) planted with native wetland emergent plants (Carex secta, Typha orientalis, Schoenoplectus tabernaemontani). Plant roots inject oxygen into the gravel matrix (rhizosphere), fostering complex aerobic and anaerobic micro-zones that absorb nutrients and degrade organics without mechanical power.

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5. Fail-Safe Electrical Monitoring, Alarms & Control Panels

Because secondary treatment systems rely on active mechanical aeration and pressure pumping, mechanical failure will quickly result in the discharge of untreated sewage. Under AS 1546.3 Clause 2.5, every secondary treatment unit must be fitted with an integrated, weather-proof audiovisual alarm and control panel.

Mandatory Monitored Failure Conditions

1. Air Blower Failure: Monitored via an in-line pneumatic pressure switch or current-sensing relay. If the air blower stalls, throws a thermal overload, or snaps its diaphragm, pressure drops below 10 kPa, triggering an immediate alarm.
2. High Water Level Alarm: An independent secondary float switch mounted in the treated effluent pump chamber. If the discharge pump trips, burns out, or the dispersal line is blocked, the rising liquid trips the float before the tank overflows.
3. Pump Motor Fault: Overload circuit breaker tripping.
4. Power Supply Loss: System circuit isolation.

Alarm Panel Features

• Audiovisual Warning: External weatherproof (IP56) enclosure mounted in a prominent, visible location (e.g. outside the garage or laundry wall). Must feature an amber or red flashing strobe beacon and a loud audible buzzer generating at least 70 dBA at 1.0 m.
• Mute Function: A push-button muting switch silences the audible buzzer for 24 hours, but the flashing visual strobe must remain illuminated until the underlying mechanical fault is physically rectified.

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6. Mandatory Maintenance Servicing Contracts & Certifying Drainlayer Obligations

Under New Zealand regional plan permitted activity rules and territorial authority building consents, secondary wastewater treatment plants have mandatory ongoing compliance obligations.

Statutory Maintenance Requirements

• Mandatory Service Contract: Building Consent Authorities (BCAs) will not issue a Code Compliance Certificate (CCC) for an AWTS installation without an executed, legally binding continuous maintenance contract between the property owner and an accredited wastewater servicing agent.
• Quarterly Inspections (Every 3 Months): The accredited technician must inspect the system every 90 days and complete a formal compliance audit, recording:
  1. Dissolved oxygen (DO) levels in the aeration chamber (must be >= 2.0 mg/L).
  2. Sludge blanket depth in the secondary clarifier and primary chamber.
  3. Air blower air filter cleaning and pneumatic pressure check.
  4. Clarifier sludge return flow rate verification.
  5. Chlorine tablet replenishment or UV quartz sleeve acid wipe-down.
  6. Pump operational run hours and float switch test.
  7. Effluent clarity, color, and odor assessment.
  8. Submission of the formal inspection report to the regional council's environmental monitoring division.

Certifying Drainlayer Sign-Off & Producer Statements

The certifying drainlayer holds legal statutory responsibility for the complete physical installation up to the point of commissioning:

• Ensuring tank excavation, bedding, buoyancy ballast, and level installation comply with manufacturer specifications.
• Executing a hydrostatic water tightness test on all tank compartments to verify zero leakage.
• Installing compliant electrical ducts, ensuring IP-rated connections, and connecting alarm telemetry.
• Commissioning the system with clean water to verify diffuser bubbling, sludge return airlift operation, and pump float travel.
• Issuing the formal Producer Statement PS3 (Construction) to the council building inspector.

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7. Worked Engineering Specification: Lake Catchment Secondary System

Design Brief

A certifying drainlayer is specifying an advanced secondary on-site wastewater treatment system for a proposed 5-bedroom luxury dwelling located within the sensitive Lake Taupo catchment. Under the Waikato Regional Plan Lake Taupo Variation, the system must achieve advanced secondary treatment with total nitrogen reduction (TN <= 15 mg/L) and secondary disinfection (E. coli <= 10 cfu/100 mL) prior to discharge via pressure-compensating subsurface drip irrigation.

Step 1: Design Flow Calculation

• Design Occupancy = Bedrooms + 1 = 5 + 1 = 6 persons.
• Water Source: Unrestricted reticulated supply, fitted with standard water fixtures (q = 200 L/person/day).
• Daily Design Flow:
  Qd = 6 persons * 200 L/p/d = 1,200 L/day

Step 2: System Selection & Chamber Capacities

The drainlayer specifies an approved precast concrete multi-chamber AWTS certified to AS 1546.3 Class A advanced secondary with nutrient reduction:

• Primary Sedimentation / Anoxic Chamber: Minimum 3,000 L capacity (provides 2.5 days retention plus sludge storage; receives continuous denitrifying sludge return).
• Aeration Chamber: Minimum 2,400 L capacity, fitted with 450 m2/m3 specific area fixed biomedia, driven by an 80 L/min linear diaphragm blower with twin fine-bubble membrane diffusers (DO >= 2.5 mg/L).
• Secondary Clarifier: 900 L hopper-bottom cone with airlift sludge return recycling at 150 L/hour back to the primary chamber.
• Disinfection Chamber: Fitted with an in-line closed-channel germicidal UV disinfection unit delivering a minimum UV fluence of 40 mJ/cm2 at peak flow.
• Pump Chamber: 1,200 L chamber housing a multi-stage stainless steel submersible pump delivering 45 L/min at 32 m head, controlled by dual float switches.
• Control Panel: Exterior IP56 panel with low-pressure blower switch, high-level pump float alarm, flashing strobe, 70 dBA siren, and telemetry module.

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8. Trade Traps & Common Certification Pitfalls

• The Chemical Shock Mass-Kill Trap: The homeowner uses heavy doses of quaternary ammonium disinfectant, acidic toilet bowl cleaners, or commercial caustic drain openers. The harsh chemicals sterilize the living biomass in the aeration chamber. Within 48 hours, the zoogloeal biofilm dies, the aeration tank turns dark grey and foams violently, thick pungent hydrogen sulfide odors erupt, and the dead biomass washes through the clarifier into the pump chamber.
• The Swimming Pool Chlorine Tablet Explosion Trap: When the erosion chlorinator runs out of tablets, the homeowner purchases cheap swimming pool chlorine tablets (trichloroisocyanuric acid / "trichlor") from a hardware store instead of approved wastewater tablets (calcium hypochlorite). Trichlor is designed for large volumes of clean, acidic pool water; when placed in a confined wastewater chlorinator rich in organic matter and ammonia, trichlor undergoes a violent exothermic reaction, releasing explosive nitrogen trichloride (NCl3) gas that shatters the plastic chlorinator canister and causes severe chemical burns.
• The Bach Power-Down Shutoff Trap: Owners leaving a holiday home (bach) for the winter switch off the master electrical board at the front door to save power. Cutting power to the AWTS shuts down the air blower. Within 12 to 24 hours, the aeration chamber exhausts its dissolved oxygen and turns completely anaerobic. The aerobic bacteria suffocate and die. When the owners return at Christmas and flip the power back on, the sludge return and aeration stir up massive volumes of foul, putrid black sludge that immediately blankets the secondary clarifier, trips the high-level float, and pumps unclarified sludge into the drip irrigation network, permanently ruining the emitter lines.