How to Write an STP Tender Specification: Scope, Performance Guarantees & Evaluation

A poorly written STP tender specification is one of the most common reasons facilities end up with a plant that underperforms within two years of commissioning. When the scope is ambiguous, the technology is left open, and performance is not contractually guaranteed, vendors bid to win — not to deliver. You get the lowest-cost system that technically clears the spec on paper, and then spend years troubleshooting outlet quality, odour complaints, and sludge disposal problems.

This guide covers every section of a robust STP tender specification — from design basis to evaluation methodology. It is written for facility managers, RWA committee members, and project consultants who are procuring sewage treatment plants for residential complexes, commercial buildings, hotels, and institutions in India.

Defining the Design Basis

The design basis is the foundation of the entire specification. Every vendor will size their plant from this section, so any error or omission here propagates into every downstream design decision. It must be unambiguous and complete.

Sewage flow calculation: Use per-capita generation rates appropriate to your building type. The Bureau of Indian Standards and CPHEEO guidelines provide the following benchmarks:

Peak flow factor: The STP must handle instantaneous peak flows that are significantly higher than the average daily flow. Specify a peak flow factor of 2.5–3× the average daily flow for residential buildings (morning and evening rush), and 2× for commercial buildings with more spread-out water usage. All hydraulic elements — screens, pumps, collection tanks, piping — must be sized to peak flow. Biological reactor sizing uses average daily flow. This distinction is critical and frequently missed.

Inlet characteristics: State the expected raw sewage quality the plant must handle. Typical domestic sewage in India ranges: BOD 200–300 mg/L, COD 350–600 mg/L, TSS 200–350 mg/L, TKN 30–50 mg/L, pH 6.5–8. If your site has kitchen waste contributions (restaurants, canteens), increase BOD to 400–600 mg/L. Do not let vendors assume their own inlet characteristics — this affects biological reactor volume and aeration capacity significantly.

Outlet standard: State the exact outlet quality requirement explicitly — do not write "as per applicable norms." For discharge to a municipal drain, specify CPCB general discharge standards: BOD <30 mg/L, TSS <30 mg/L, pH 6.5–9. For treated water reuse (toilet flushing, horticulture, cooling tower makeup), specify the CPCB reuse standard: BOD <10 mg/L, TSS <10 mg/L, Total Coliform <100 MPN/100 mL. This outlet standard will determine the technology — specifying a reuse standard effectively mandates MBR or a robust tertiary treatment train.

Future expansion: Specify that the civil structure must be designed for 1.3× the current design flow, with provisions (sleeves, blower capacity headroom, panel space) for future capacity addition. This costs very little upfront and avoids a major civil rework in 5–7 years.

Scope of Work — What to Include

An STP tender scope that simply says "supply, install, and commission STP of X KLD capacity" will generate 10 different quotes covering 10 different scopes. Every item below should be explicitly included or excluded — ambiguity always costs money later.

Civil and structural works: Specify whether civil tanks (equalization tank, aeration reactor, sludge sump) are in scope or provided by the civil contractor. If the STP vendor is responsible, include RCC tank construction, waterproofing, grating, handrails, access platforms, and covers. If civil is excluded, provide tank dimensions and loading assumptions to the civil contractor.

Mechanical equipment: Bar screen (coarse + fine), grit chamber, equalization tank mixing and transfer pumps, aeration system (blowers + diffusers), biological reactor internals (media, membrane modules, or SBR decanter as applicable), secondary clarifier (if not MBR), tertiary treatment (sand filter, UV, or chlorination), sludge thickener and dewatering (filter press or sludge drying bed), and all interconnecting pipework within the STP boundary.

Electrical panel and instrumentation: Main distribution board, motor control centre, DO sensors in reactor, level transmitters in all sumps, flow meter on inlet and outlet, pressure gauges on blower discharge, and all wiring within the STP battery limit. Specify that all instruments must be 4–20 mA with HART protocol, compatible with SCADA integration.

Treated water reuse distribution: If treated water is to be reused within the complex (flushing tanks, horticulture points, cooling tower makeup), specify the treated water transfer pump, storage tank, and distribution header up to the complex boundary. This is frequently omitted and becomes a post-award dispute.

SCADA and remote monitoring: Mandate a SCADA system with real-time monitoring of all key parameters — inlet flow, outlet DO, pH, treated water quality — with remote access capability and daily automated reports. Many State PCBs now require online continuous effluent monitoring for STPs above a certain capacity. Build this in from the start.

Commissioning and handover: Specify that commissioning includes seeding the bioreactor, achieving stable biological performance, and demonstrating outlet quality against the performance guarantee before final handover. Include operator training (minimum 3 days), O&M manual in English and local language, and as-built drawings in AutoCAD format.

O&M period: Include a mandatory 12-month post-commissioning O&M period in the contract, with the vendor responsible for plant operation, routine maintenance, consumable supply, and outlet quality. This creates a strong incentive for the vendor to deliver a robust plant — they will be the ones operating it.

Performance Guarantee Clauses

This is the most important — and most frequently inadequate — section of STP specifications. Performance guarantees must be contractual obligations with specific numbers, a defined test protocol, and real consequences for non-conformance. Vague language like "the STP shall meet CPCB norms" is not a performance guarantee.

Specific outlet parameters: State these as numbered contractual obligations, not design targets. For a reuse-grade STP:

• BOD: <10 mg/L (daily composite sample)
• COD: <50 mg/L
• TSS: <10 mg/L
• Total Coliform: <100 MPN/100 mL
• pH: 6.5–8.5
• Turbidity: <5 NTU (if cooling tower makeup is in scope)

For a discharge-grade STP (outlet to drain), the parameters are BOD <30 mg/L, TSS <30 mg/L, pH 6.5–9 — matching CPCB general standards. In both cases, the numbers should appear in the contract document, not just in the specification.

Acceptance test protocol: Specify a formal performance acceptance test as a condition for release of the final payment milestone. The protocol must define: duration (72-hour continuous operation at design flow), sampling (composite samples every 4 hours), analysis by an NABL-accredited third-party laboratory (not the vendor's internal lab), and parameters to be tested. The STP vendor shall bear the cost of the acceptance test.

Consequence of non-conformance: If the acceptance test fails, the vendor must rectify at their cost and repeat the test. Final payment (typically 10% of contract value) is withheld until the performance guarantee is demonstrated. If the vendor fails to meet the guarantee within 6 months of initial commissioning, the client has the right to invoke the performance bank guarantee. State the bank guarantee amount — 10% of contract value is standard.

Warranty scope: Distinguish between equipment warranty and performance warranty. Equipment warranty (defects in materials and workmanship) for 12 months from commissioning is standard and usually offered voluntarily. The performance warranty — guarantee that the plant will continue to meet outlet standards — must be explicitly demanded. Without it, a vendor can claim the plant "worked fine" while your outlet is out of spec due to a design inadequacy rather than a hardware failure.

Equipment Specifications That Matter

Not every equipment item needs a detailed specification. Focus your specification effort on the items that most affect long-term performance and operating cost.

Aeration system: Specify whether coarse-bubble or fine-bubble diffusers are required. For biological reactors, mandate fine-bubble membrane disc diffusers with a Standard Oxygen Transfer Efficiency (SOTE) of minimum 20% at design submergence. Specify blower type (roots/rotary lobe vs screw type vs turbo blower), motor efficiency class (IE3 minimum), and installed capacity with 25% standby. The aeration system is the single largest power consumer — a poor specification here will cost lakhs per year in excess power.

Screen size and type: Specify a mechanically cleaned bar screen (not manual) with bar spacing of 6 mm or less for the fine screen. For plants above 100 KLD, specify a rotary drum screen or channel band screen. Manual screens may seem cheaper but they are rarely cleaned properly and cause downstream equipment problems.

MBR membrane specifications (if MBR technology is specified): State the membrane type (hollow fibre or flat sheet), nominal pore size (0.04–0.1 micron), membrane material (PVDF preferred for chemical resistance), design flux (10–15 LMH for hollow fibre), and minimum design membrane life (7 years under normal operating conditions). Require the vendor to provide type-test reports from an independent laboratory for membrane integrity and performance. Also specify the membrane cleaning system — CIP (clean-in-place) with chemical dosing ports and automated maintenance clean cycle.

Sludge dewatering: Specify the target sludge cake dry solids content (minimum 18–22% DS for filter press, 15–18% DS for screw press) and the design sludge generation rate. Sludge disposal is an ongoing OPEX item — a poorly dewatered sludge (10% DS vs 20% DS) doubles the volume and cost of disposal. Require the vendor to specify sludge generation in kg DS/day and dewatered cake volume in m³/week.

UV disinfection system: If UV is specified for disinfection, state the minimum UV dose of 30 mJ/cm² (for reuse applications, 40 mJ/cm²), lamp type (low-pressure amalgam preferred for energy efficiency), and auto-cleaning mechanism for the quartz sleeve. Specify ballast type (electronic, not magnetic) and require validation test reports per NSF 55 or equivalent.

SCADA I/O list: Include a minimum I/O list in the specification so that vendors price an equivalent system. As a minimum, specify: inlet flow (AI), outlet flow (AI), equalization tank level (AI), reactor DO (AI), outlet pH (AI), outlet turbidity (AI for reuse plants), blower status (DI), pump run/trip status (DI for all pumps), auto/manual mode status (DI), and MCC alarm summary. Without a defined I/O list, SCADA scope is interpreted very differently by different vendors.

Tender Evaluation Methodology

The evaluation methodology determines which vendor wins — and therefore, which plant you get. A pure lowest-cost evaluation for STP procurement is almost always the wrong approach.

Total cost of ownership: Require vendors to submit a 10-year total cost of ownership (TCO) estimate alongside their capital cost. The TCO should include annual power cost (kWh/m³ × design flow × power tariff), annual chemical cost (coagulant, disinfectant, cleaning chemicals), annual consumables (UV lamps, filter media, membrane modules if MBR), and annual O&M manpower (operator cost for two shifts). For an MBR plant, membrane replacement every 5–8 years is a major TCO item — a vendor who offers a low CAPEX on expensive proprietary membranes may be the most expensive over 10 years. Evaluate TCO = CAPEX + NPV of 10-year OPEX at a 10% discount rate.

Technical score vs commercial score (60:40 recommended):

Reference site visit: Shortlist the top two technical bidders and require a site visit to a reference installation before award. Ask to see the actual outlet water quality test reports from the reference site — not a brochure, not a schematic. A plant that cannot demonstrate a functioning reference site of similar capacity is a significant risk. Make this a mandatory pre-award step, not optional.

Two-envelope system: Collect technical and commercial bids in separate sealed envelopes. Open and evaluate technical bids first. Only open commercial bids from technically compliant vendors. This prevents commercial pressure from distorting technical evaluation.

Common Specification Gaps That Lead to Problems

Based on reviewing STP procurement documents from residential complexes and commercial buildings across India, these are the gaps that most consistently lead to underperforming plants:

Missing peak flow factor: The single most common civil design error. The STP is sized for average daily flow, screens and collection sumps overflow during morning peak, and raw sewage bypasses treatment. Always specify peak flow separately and mandate that hydraulic elements are sized to peak.

No performance test protocol: The specification states outlet quality requirements but provides no test methodology. The vendor hands over the plant after a one-day commissioning run, claims it is working, and final payment is released. Six months later, the outlet is out of spec. Without a contractual test protocol, there is no basis for withholding payment or invoking the performance guarantee.

Technology left completely open: "The vendor may propose any suitable technology" sounds flexible but guarantees non-comparable bids. You cannot evaluate an MBR proposal against a trickling filter proposal on the same criteria. Specify the technology category, or publish a functional specification with outlet quality requirements that implicitly constrains the technology choice.

No sludge handling specification: The specification covers treatment of sewage but is silent on sludge. The vendor installs no dewatering equipment, and the plant generates 2–4 m³/day of wet sludge (3–5% DS) with no disposal route. Within months, the sludge sump overflows and the plant is overwhelmed. Always specify sludge thickening, dewatering, and disposal responsibility explicitly.

No SCADA requirement: Manual operation of an STP without any monitoring instruments means problems are detected only when the outlet is visibly dirty or when a PCB inspection occurs. Instrument and SCADA requirements are frequently cut from budget without understanding that they are the primary tool for maintaining compliance and optimising operating cost.

No O&M training provision: The STP is commissioned by the vendor's engineer, who then leaves. The in-house operator has never been trained on the specific plant configuration. Within weeks, aeration is running continuously (wasting power) or not running enough (causing odour), and the operator has no baseline to diagnose problems. Mandate formal operator training with a sign-off checklist as a commissioning deliverable.

Warranty covering only equipment, not performance: Standard equipment warranties cover manufacturing defects for 12 months. They do not cover the scenario where the plant was correctly built but incorrectly designed — undersized aeration, wrong diffuser selection, inadequate sludge age. A performance warranty is what closes this gap. It must be explicitly negotiated into the contract. Without it, design inadequacies are at your cost, not the vendor's.