Microcontroller-Based pH and TDS Monitoring Panel for a Model Effluent-Treatment Plant

Microcontroller-Based pH and TDS Monitoring Panel for a Model Effluent-Treatment Plant is an instrumentation project that continuously displays the three numbers a small dyeing or processing unit's effluent-treatment plant (ETP) lives and dies by — pH, total dissolved solids (TDS) and temperature — and raises alarms the moment a reading slips outside the discharge norms the unit's TNPCB consent prescribes.

The panel is built on the AT89S52 microcontroller from the M-scheme Microcontroller course. A pH glass electrode, a conductivity/TDS probe and an LM35 feed an ADC0809 through conditioning circuits; firmware in Embedded C converts the counts to engineering units, applies temperature compensation, and shows the readings on a 16x2 LCD. A green/red lamp pair and a buzzer indicate compliance; a relay drives a small peristaltic pump that demonstrates acid dosing when the sample tank goes alkaline — the classic dyeing-effluent problem.

Because a pH electrode is only as good as its last calibration, the project includes proper buffer calibration (pH 4, 7 and 9.2 sachets) and TDS calibration against standard solutions — the same routine an ETP operator follows on site. The whole panel is first simulated in Multisim / TINA, then built on a veroboard and demonstrated on a bench rig of sample tanks, using safe synthetic effluent instead of real trade effluent.

Small dyeing and processing units must discharge effluent within strict pH and TDS limits, yet most of them verify compliance by occasional manual sampling with handheld meters that are rarely calibrated. Between two checks an alkaline or high-salt surge passes unnoticed, and the first person to notice is usually the pollution-control inspector — by which time the unit faces a show-cause notice, or worse, a closure direction. Operators also lack temperature data, which affects both the pH reading itself and the process records. There is a clear need for a fixed, low-cost monitoring panel that measures pH, TDS and temperature continuously, displays them in engineering units, alarms against the consent limits, and can trigger a corrective action such as acid dosing — at a cost a small unit can afford and a diploma team can build.

Abstract

Small textile dyeing and processing units in Tamil Nadu operate under strict effluent norms, and their ETPs fail audits most often for the simplest reason: process parameters are checked occasionally, by hand, with a portable meter that may itself be uncalibrated. This project builds a fixed monitoring panel that continuously measures pH, TDS and temperature of the treated effluent, compares them against consent limits, and alarms before an out-of-spec discharge happens. It is built around the AT89S52, ADC0809 and low-cost probes, and demonstrated on a safe bench rig.

Introduction

Tamil Nadu's dyeing clusters, from Tirupur to Erode, have been through closures and zero-liquid-discharge mandates precisely because untreated or badly treated effluent damages rivers and farmland. A unit that installs common or in-house ETP machinery still depends on knowing, hour by hour, what the water leaving it actually contains. The panel this project builds gives that visibility at a parts cost under ₹3,000, using the microcontroller, sensor-interfacing and simulation skills the diploma curriculum already teaches.

Existing System

• Operators spot-check pH with a handheld pen meter or litmus paper a few times a shift; TDS is checked with a pocket meter or not at all.
• Readings are written in a register when someone remembers; there is no alarm between checks, so an alkaline surge is discovered at the next sampling — or at the board's inspection.
• Handheld meters drift and are rarely buffer-calibrated, so even the recorded values are questionable.
• Limitations: no continuous indication, no audible alarm, no temperature data for compensation or records, no automatic response such as dosing.

Proposed System

• Continuous pH measurement with a glass electrode and conditioning board, calibrated at pH 4, 7 and 9.2 buffers, displayed to 0.1 pH.
• Continuous TDS measurement through a conductivity probe, with readings shown in ppm and calibrated against a standard solution.
• Temperature measurement with an LM35, used both for display and for temperature compensation of the pH reading.
• Norm alarms: green lamp inside limits, red lamp plus buzzer when pH leaves the 5.5–9.0 band (CPCB general discharge standard) or TDS crosses the consent limit (2,100 ppm as our working value).
• Automatic dosing demo: when the sample tank exceeds pH 9, a relay runs a peristaltic pump dosing dilute acid until the pH returns inside a deadband — a safe, scaled demonstration of neutralisation control.
• Error codes for a dry or unplugged electrode, verified by pulling the probe in the rig.

Feasibility Study

• Technical: electrode interfacing, ADC use, LCD displays and relay driving are all within the 34052 and 34066 course work; the conditioning circuits are simulated in Multisim / TINA before purchase.
• Economic: the bill of materials is about ₹2,300–₹2,800 including the pump and probes, versus thousands for a commercial online analyser.
• Operational: calibration takes ten minutes with standard buffers and solutions — the exact routine an operator already knows — and the panel runs from a 12 V adapter.

In scope

• A bench demo rig: three sample tanks (alkaline, high-TDS, neutral) made from safe synthetic materials — diluted detergent and common salt — never real trade effluent.
• Continuous pH, TDS and temperature display; norm alarms; peristaltic-pump dosing demonstration on the alkaline tank.
• Three-point pH calibration, TDS calibration, temperature-compensation routine and probe-fault detection, each with recorded test results.

Out of scope (v1)

• No BOD/COD/SS measurement — those need lab analysis beyond a bench panel; we log them as future work.
• No GSM/SCADA link to the board's systems; data logging is manual from the LCD in v1.
• No real trade effluent handling, and no dosing of concentrated acids — the demo uses dilute solution and small volumes with gloves and goggles.
• No zero-liquid-discharge machinery design; the panel monitors, it does not treat.

The project follows a phased build-and-test plan with hardware milestones matched to the M-scheme reviews: Review I after design and simulation, Review II after the working breadboard panel.

Testing approach: every channel is validated against a reference — buffers for pH, a standard solution and a handheld meter for TDS, a lab thermometer for temperature. Threshold behaviour is tested at the band edges (5.4/5.6 pH, 8.9/9.1 pH, 2,050/2,150 ppm), the dosing deadband is verified for overshoot, and probe-fault codes are triggered deliberately. Every case goes into the report's expected-vs-observed table.

The panel is organised as three sensing channels, one conversion-and-control core, and an output stage — a conventional instrumentation layout the examiners can follow at a glance.

flowchart LR
  A["12 V adapter"] --> PS["5 V regulated supply (7805)"]
  P["pH glass electrode"] --> PC["pH conditioning board (high-impedance buffer)"]
  T["Conductivity/TDS probe"] --> TC["TDS module (oscillator + rectifier)"]
  M["LM35 in sample tank"] --> MC["RC filter"]
  PC -->|"CH0"| AD["ADC0809, Vref 2.56 V"]
  TC -->|"CH1"| AD
  MC -->|"CH2"| AD
  PS --> AD
  AD -->|"8-bit data, EOC handshake"| U["AT89S52 (8051)"]
  PS --> U
  U --> L["16x2 LCD"]
  U --> GL["Green lamp"]
  U --> RL["Red lamp + buzzer"]
  U --> DR["ULN2003 + relay"] --> PM["12 V peristaltic dosing pump"]

pH channel. A glass electrode develops a tiny Nernst voltage (about 59 mV per pH unit at 25 °C) at extremely high impedance, so the conditioning board's first stage is a high-input-impedance buffer; its output spans the ADC's 2.56 V window. Because electrode slope drifts with age and temperature, the firmware applies a two-point slope correction from the last buffer calibration, plus an LM35-based compensation term.

TDS channel. The conductivity module drives the probe with an AC excitation (so electrodes do not polarise), rectifies the result, and hands an analogue voltage to CH1. The firmware converts counts to ppm using the calibration factor and displays it alongside pH.

Control core. Firmware in Embedded C cycles through the three channels with the standard ALE/START/EOC handshake, averages eight samples, and refreshes the LCD every 500 ms.

flowchart TD
  S["Main loop (every 500 ms)"] --> R["Read pH, TDS and temperature; average 8 samples"]
  R --> C["Convert with calibration + temperature compensation"]
  C --> D["Update LCD and lamp states"]
  D --> Q{"pH above 9.0?"}
  Q -->|Yes| P1["Run dosing pump; red lamp + buzzer"]
  Q -->|No| Q2{"Inside 5.5 to 9.0 and TDS within limit?"}
  Q2 -->|Yes| G["Green lamp steady"]
  Q2 -->|No| P2["Red lamp + buzzer (no dosing for TDS)"]
  P1 --> Q3{"pH below 8.5 (deadband)?"}
  Q3 -->|Yes| S
  Q3 -->|No| P1
  G --> S
  P2 --> S

Output stage. The dosing pump runs only while the pH is above 9.0 and stops when it falls below 8.5 — a deadband that prevents the pump from hunting around the set point, exactly as an on-off neutralisation loop behaves on a real ETP.

• Add a microSD or EEPROM data logger so the unit keeps an hour-by-hour compliance record for inspections.
• A GSM module that SMSes the operator when an alarm persists beyond a set time.
• BOD/COD estimation support through timed sampling of the rig's outlet for lab analysis.
• Flow measurement with a paddle-wheel sensor so the panel computes pollutant load, not just concentration.
• A PID-style proportional dosing loop instead of on-off control, for smoother neutralisation.
• A weatherproof field enclosure and solar-backed supply for mounting at the actual ETP outflow point.