Tertiary Treatment Plant (TTP):
Process, Technologies, Cost Drivers and Industrial Reuse

What is a Tertiary Treatment Plant?

For an industrial facility, the objective is to produce water that can reliably replace freshwater in a specified application — at a sustainable operating cost.

The starting point for TTP design is therefore the reuse requirement: where will the recovered water go, and what quality does that application need?

What is the difference between a TTP, STP and ETP?

An STP treats sewage. An ETP treats industrial effluent. A TTP describes a further treatment stage that may follow either system.

How does a tertiary treatment plant work?

A TTP removes or controls the residual contaminants that prevent treated wastewater from being used in its intended application.

A membrane-based industrial reuse scheme may include:

1. Feed balancing: Manage variations in flow and water quality.
2. Feed-specific conditioning: Address contaminants such as hardness, residual organics or scale-forming constituents where necessary.
3. Filtration or ultrafiltration: Reduce suspended solids and colloidal material.
4. Reverse osmosis, where required: Reduce dissolved salts to meet the reuse specification.
5. Final conditioning and storage: Provide appropriate disinfection, pH adjustment or further polishing before distribution.

This is a possible treatment sequence, not a standard equipment list for every plant. Conditioning may occur at different points, and some applications do not require RO.

DuPont's industrial wastewater guidance distinguishes between UF for suspended solids and colloids, RO for dissolved salts, and targeted ion-exchange treatment for hardness or alkalinity. These processes perform different functions within the treatment train. ^[1]

Which technologies are used in a TTP?

Technology selection should follow the contaminants to be controlled and the required product-water quality.

UF can provide a useful particulate barrier ahead of RO. However, low turbidity alone does not demonstrate that water is suitable for high-recovery RO: dissolved scaling constituents and residual organic fouling potential must also be assessed. ^{[1], [2]}

Does every tertiary treatment plant need RO?

For some applications, filtration and appropriate disinfection may be sufficient. Others require substantial reduction of salinity, hardness, silica or specific dissolved contaminants.

Including RO without a defined need introduces additional power consumption, membrane maintenance and concentrate management. Omitting it where dissolved-water quality demands it can transfer the problem into the user's cooling, process or utility systems.

The appropriate decision requires a comparison of:

• Actual treated-effluent quality and its variability.
• Required quality at the reuse point.
• Available blending opportunities.
• Sustainable recovery and residual-stream management.
• Total cost of producing and delivering usable water.

Where can tertiary-treated water be reused?

Industrial reuse destinations can include cooling-tower makeup, selected process operations, washing and feed to a boiler-makeup treatment system. Each destination requires its own acceptance criteria.

Cooling-tower makeup

Assess the proposed makeup water against the circulating-water chemistry and intended cycles of concentration. Review scaling, corrosion, fouling and microbiological control together. Water that appears suitable as makeup may become problematic after dissolved constituents concentrate in the cooling circuit.

Process water

Set the water-quality specification against the manufacturing operation. Conductivity, hardness, colour, organics and specific ions may affect production differently across industries. A specification suitable for one process should not automatically be transferred to another.

Boiler-makeup treatment

Recovered water may serve as feed to a dedicated boiler-makeup treatment system. It should not automatically be described as directly suitable for boiler feed. Further demineralisation or polishing may be necessary, depending on the boiler pressure and equipment requirements. RO and ion exchange are established components of boiler-water treatment, with the complete treatment train selected for the required duty. ^[3]

Washing, flushing and other non-potable uses

These applications still require defined quality, microbial controls where relevant, and appropriate storage and distribution arrangements. A TTP designation alone does not establish that its product water is potable.

What information is needed to design a TTP?

A reliable design requires representative feed analysis, a defined reuse specification and an operating water balance.

A single favourable sample may not represent operating conditions. Sampling should capture relevant production changes, cleaning discharges and upstream treatment variability.

For membrane systems, the assessment should establish a sustainable design flux, recovery, cleaning strategy and expected net production. UF operation itself includes cleaning and backwash cycles that must be accommodated in plant design. ^[4]

How should TTP capacity and recovery be specified?

Specify both feed capacity and net reusable-water output.

A procurement specification should distinguish:

• Feed flow in m³/h.
• Net reusable-water production in m³/day.
• Product-water quality.
• Overall recovery across the defined plant boundary.
• Operating hours and required availability.
• Quantities and destinations of residual streams.

What determines the cost of a tertiary treatment plant?

TTP cost depends on the treatment duty and project scope — not capacity alone. Two plants with the same feed flow may require substantially different equipment and operating expenditure.

Capital-cost drivers

The main drivers include feed quality, required product quality, pretreatment complexity, membrane configuration, redundancy, automation, civil works, materials of construction and integration with existing facilities.

Storage, distribution piping, electrical infrastructure and concentrate handling should be clearly included or excluded when comparing proposals.

Operating-cost drivers

Assess electricity, chemicals, cleaning, membrane and media replacement, manpower, maintenance, sludge management and concentrate treatment.

A useful comparison is:

Use water meeting the agreed specification as the denominator. For a full lifecycle comparison, also account for capital recovery and major replacements.

The economic assessment should consider the freshwater purchases or abstraction costs displaced, additional distribution costs and any demonstrable change in downstream treatment expenditure.

Can an existing STP or ETP be upgraded with a TTP?

Yes, provided the existing outlet and infrastructure can support the proposed reuse duty.

The assessment should establish whether the main constraint lies in:

• Unstable upstream treatment.
• Residual solids or colloids.
• Dissolved salts or scaling constituents.
• Organic fouling.
• Insufficient hydraulic capacity.
• Storage, utilities or reuse-water distribution.
• Concentrate and cleaning-waste management.

Adding membrane equipment cannot compensate for every upstream deficiency. In some facilities, correcting the STP or ETP operation is the first requirement for a reliable reuse project. A retrofit should therefore be evaluated as an integrated plant modification.

How does a TTP support zero liquid discharge?

A TTP can prepare wastewater for membrane recovery and enable more water to be reused before the remaining concentrate reaches thermal treatment.

Where additional membrane recovery is technically and economically feasible, it can reduce the volume sent to an evaporator. The implications for thermal duty depend on the resulting brine chemistry and equipment operating conditions.

What relevant TTP experience does SR Paryavaran bring?

SR Paryavaran Engineers (P) Ltd.'s relevant project experience includes:

These references provide context for evaluating industrial reuse and integration between tertiary treatment, membrane recovery and downstream systems. The appropriate configuration for another facility must be established from its own water quality, reuse demand and operating constraints.

Frequently asked questions