Wastewater Parameters Explained: TDS, pH, TSS, and Heavy Metals

Every ETP lab report lists a set of parameters against CPCB limits, but the reasoning behind why each parameter is measured — and what it reveals about your wastewater and treatment process — is rarely explained. This guide covers the parameters that appear on virtually every industrial effluent monitoring report: pH, TSS, TDS, heavy metals, oil & grease, and ammoniacal nitrogen, along with the sector-specific additions that appear under individual Consent to Operate conditions.

Why These Parameters Appear on Every Lab Report

The CPCB General Standards under the Environment (Protection) Rules, 1986 specify 18+ parameters that apply to all industrial discharges to inland surface waters. These parameters can be grouped into four categories:

• Physical: TSS, TDS, colour, temperature, turbidity
• Chemical: pH, BOD, COD, oil & grease, phenols, ammoniacal nitrogen
• Inorganic trace elements: heavy metals — lead, mercury, cadmium, chromium, arsenic, copper, zinc, nickel, and others
• Microbiological: coliform counts, relevant primarily for STPs and treated water reuse

For most ETPs, the core panel tested at each monitoring event is: pH, BOD, COD, TSS, TDS, oil & grease, ammoniacal nitrogen, and any sector-specific metals required under the CTO. Understanding what each parameter represents helps operators identify the source of a failure and the appropriate corrective action — rather than treating each out-of-limit result as an isolated problem to report and move on from.

For a detailed overview of how these tests are conducted in the laboratory and which IS methods apply, see the guide to third-party testing and CPCB/SPCB requirements.

pH: The Master Variable in Wastewater Treatment

pH is the measure of hydrogen ion concentration on a logarithmic scale from 0 (very acidic) to 14 (very alkaline). The CPCB General Standard for inland surface water discharge is pH 6.5–8.5. This narrow window reflects the tolerance range of aquatic life in receiving water bodies, where pH outside this range disrupts fish respiration, reproduction, and the broader aquatic ecosystem.

pH is one of the most operationally significant parameters in an ETP because it affects nearly every treatment process:

• Biological treatment performance: The optimal pH range for most heterotrophic bacteria in activated sludge and MBBR systems is 6.5–8.0. Outside this range, biological treatment effectiveness drops sharply — below pH 6.0, bacterial activity is severely inhibited; above pH 9.0, nitrification is particularly affected.
• Chemical treatment: DAF (Dissolved Air Flotation), coagulation with alum or ferric chloride, and chemical precipitation of metals are all highly pH-sensitive. Coagulation with alum is most effective at pH 6.5–7.5; ferric coagulants work better at pH 5.5–7.0.
• Metal solubility and toxicity: Metals precipitate as insoluble hydroxides at elevated pH, making high pH a tool for metal removal. At low pH, many metals become more soluble and therefore more mobile and toxic.

pH must be measured on-site immediately after sample collection — it changes during sample storage due to off-gassing of CO₂, microbial activity, and temperature shifts. Testing method: IS 3025 Part 11, using a calibrated pH electrode with buffer calibration at pH 4.0, 7.0, and 10.0.

Common sources of pH exceedance in different industries:

TSS and TDS: What Each Measures and Why

TSS and TDS are often confused because they share the word "solids," but they measure fundamentally different fractions of wastewater content.

TSS — Total Suspended Solids

TSS is the mass of solid particles retained on a 1.2 µm glass fibre filter (Whatman GF/C) when a measured volume of sample is filtered and the filter is dried at 105°C. Testing method: IS 3025 Part 17. CPCB limit for inland surface water: ≤100 mg/L.

In ETP effluent, TSS originates from: sludge carryover from secondary clarifiers (the most common cause), inadequate sand or pressure filtration at the tertiary stage, and biological sludge in poorly settling bulking sludge conditions where filamentous bacteria dominate. High TSS also contributes directly to BOD and COD readings — suspended organic solids consume oxygen in the BOD test and are oxidised in the COD test. A TSS failure almost always means elevated BOD and COD as well.

Quick operational check: hold a 250 mL glass sample to natural light. Visible turbidity or particulates visible to the naked eye typically indicate TSS well above 50 mg/L.

TDS — Total Dissolved Solids

TDS is the dissolved material passing through the 1.2 µm filter — the filtrate from the TSS test. It is measured gravimetrically by evaporating a known volume at 180°C and weighing the residue (IS 3025 Part 16), or estimated from electrical conductivity (EC × 0.64 as a rough conversion factor). CPCB limit for inland surface water: ≤2,100 mg/L.

TDS includes dissolved inorganic salts (sodium, calcium, magnesium, chlorides, sulphates, bicarbonates), dissolved organic compounds, and trace elements. TDS is particularly important for:

• Treated water reuse: High TDS limits agricultural use (crop tolerance varies by species) and process water reuse (scaling in heat exchangers, interference with product quality).
• ZLD system design: TDS determines the evaporator load and concentrate volume in Zero Liquid Discharge plants.
• High-salt industries: Food processing (brine, pickling), chemical manufacturing, and tanneries generate high-TDS wastewater that requires specific treatment pathways.

TDS cannot be reduced by conventional biological treatment — primary, secondary, and even tertiary biological processes leave dissolved salts unchanged. TDS reduction requires membrane processes (reverse osmosis or nanofiltration) or thermal evaporation for ZLD compliance.

Heavy Metals in Industrial Wastewater

Heavy metals are a critical concern for specific industrial sectors. Unlike organic pollutants, metals cannot be destroyed — only transferred, precipitated, or concentrated. CPCB General Standards for common metals in inland surface water discharge:

Testing method: IS 3025 Part 46 using atomic absorption spectrometry (AAS) — flame AAS for higher concentrations, graphite furnace AAS for trace levels. ICP-OES (inductively coupled plasma optical emission spectrometry) allows simultaneous multi-element analysis and is increasingly used at accredited labs for efficiency.

Standard treatment for heavy metals in ETP: chemical precipitation as metal hydroxides by raising pH to 9–11 with lime or caustic soda, followed by coagulation and clarification to remove the precipitated hydroxide floc. For more complete removal of trace metals, sulphide precipitation achieves lower residual concentrations than hydroxide precipitation. Ion exchange or reverse osmosis can polish residual metals after precipitation.

Hexavalent chromium (Cr⁶⁺) requires a separate pre-treatment step before precipitation: reduction of Cr⁶⁺ to Cr³⁺ using ferrous sulphate (FeSO₄) or sodium metabisulphite at acidic pH (2–3). Only after reduction can the Cr³⁺ be precipitated as Cr(OH)₃ by raising pH to 8–9. Attempting to precipitate Cr⁶⁺ directly by pH raise is ineffective — Cr⁶⁺ forms chromate ions (CrO₄²⁻) that remain soluble.

Oil & Grease and Ammoniacal Nitrogen

Oil & Grease (O&G)

Oil & Grease is measured by solvent extraction using n-hexane (IS 3025 Part 42). The solvent extracts both free oil and emulsified fat, which is then gravimetrically measured after solvent evaporation. CPCB limit for inland surface water discharge: ≤10 mg/L.

O&G enters ETP wastewater from: food processing operations (fats, dairy fat, frying oils), petroleum handling and storage facilities, vehicle workshops and garages, and food courts and commercial kitchen drainage. The problems created by elevated O&G are:

• Fouling of biological media surfaces in fixed-film systems (MBBR, MFBBR)
• Inhibition of biological treatment by forming a film over biomass
• Membrane fouling in MBR and RO systems
• Creation of anaerobic zones in aeration tanks, causing odour and poor treatment

Treatment: gravity oil separation (API separator) for free oil, coagulation and Dissolved Air Flotation (DAF) for emulsified oils and fats. DAF is the primary pre-treatment technology for industries with significant O&G load. O&G removal before the biological stage is essential to protect biological treatment performance.

Ammoniacal Nitrogen (NH₃-N)

Ammoniacal nitrogen is measured by Nessler's colorimetric method or titrimetric distillation (IS 3025 Part 34). CPCB limit for inland surface water: ≤50 mg/L — though many state PCBs and sensitive receiving bodies impose stricter limits of ≤10 mg/L or even ≤5 mg/L.

NH₃-N enters wastewater from: protein degradation in food processing, dairy, and slaughterhouse effluents; urea-based cleaning agents; fertiliser manufacturing runoff; and anaerobic digestion processes (ammonium is released when proteins are anaerobically broken down). In biological treatment, NH₃-N is oxidised to nitrate through nitrification — a two-step process by autotrophic bacteria (Nitrosomonas converts NH₃ to NO₂⁻; Nitrobacter converts NO₂⁻ to NO₃⁻). Nitrifying bacteria have specific requirements: sludge retention time (SRT) above 10–15 days, dissolved oxygen above 2 mg/L, and temperature above 15°C (activity drops sharply below 12°C). High NH₃-N is also acutely toxic to fish at concentrations above 0.5 mg/L — toxicity increases significantly at higher pH where the un-ionised NH₃ form predominates.

Industry-Specific Parameters Beyond the Standard Panel

Beyond the standard parameter panel, specific industries are required to test additional parameters under their Consent to Operate (CTO) conditions. These are set by the SPCB based on the nature of the manufacturing process and the pollutants associated with each sector.

For industries with complex or unusual parameter requirements, third-party testing through a NABL-accredited laboratory is mandatory for regulatory submissions. See the guide to third-party testing and CPCB/SPCB requirements for details on accreditation, sampling protocols, and submission requirements.

For biological treatment systems including MBBR, understanding which parameters your biological stage can and cannot address is essential for designing an ETP that achieves compliance across all required parameters — not just BOD and COD.