River Water Quality Buoy.

Project Overview

A self-powered floating station that logs pH, turbidity and dissolved oxygen along a river, catching pollution events that a monthly grab-sample would never see.

A river's health is usually judged from a grab sample: someone drives to a bridge once a month, dips a bottle, and sends it to a lab. It is accurate for that instant, but a river is not a constant — a factory discharge at 2 a.m., a slug of run-off after a storm, a diurnal swing in oxygen as aquatic plants photosynthesise by day and respire by night — all come and go between visits, invisible to monthly sampling. This buoy trades a little laboratory accuracy for continuous presence: it sits in the water day and night, logging the key water-quality signals, so a pollution event that lasts hours is captured with its timing and magnitude instead of being missed entirely.

It measures the three parameters that reveal most about a river cheaply and continuously: dissolved oxygen, the master variable for aquatic life, whose crashes signal pollution or eutrophication; pH, which shifts with industrial discharges, acid run-off and biological activity; and turbidity, the cloudiness that tracks sediment, run-off and many effluents and is often the first visible sign something has entered the water. Water temperature is logged alongside because it governs oxygen solubility and biological rate, and EC/TDS optionally tracks dissolved salts and pollution load. Together these paint a live picture of the river's state and, crucially, its changes.

Surviving in a river is its own engineering problem, and the design takes it seriously: the buoy is solar-powered because there is no mains in a river, communicates over LoRa or cellular because there is no Wi-Fi, is sealed and ruggedised against constant immersion, fouling and debris, and logs locally so a lost link never loses data. It timestamps everything, flags sensor faults and fouling, and raises an alert when a parameter crosses into a danger zone or changes abruptly — the signature of a discharge event. It cannot replace a certified lab, but it can tell you when to send someone with a bottle, which is often the difference between catching a polluter and cleaning up after them.

What this project does

• Continuously logs dissolved oxygen, pH, turbidity and water temperature
• Optionally logs EC/TDS as a dissolved-pollution-load proxy
• Timestamps all data and detects abrupt changes that signal discharge events
• Alerts when a parameter enters a danger zone (e.g. DO crash, pH swing)
• Runs on solar + battery as a moored floating station
• Reports over LoRa or cellular and logs locally through link outages
• Flags sensor faults and biofouling rather than reporting bad data

Real-World Applications

Deployment contexts where a build of this kind earns its keep.

Features & Capabilities

• Continuous presence that catches transient pollution events
• DO, pH and turbidity — the highest-value continuous river signals
• Change-detection for discharge events, not just fixed thresholds
• Temperature-compensated DO and pH
• Rugged, sealed, solar, moored design for life in a river
• Local logging + LoRa/cellular for remote reaches
• Fouling/fault flags to keep the record honest

Difficulty, Time & Required Skills

Skills you should have (or will pick up)

• Calibrating and temperature-compensating pH, DO and turbidity probes
• Designing a rugged, sealed, solar-powered floating platform
• Change/anomaly detection for event flagging
• LoRa/cellular telemetry with local logging fallback
• Managing biofouling and its effect on submerged sensors

Bill of Materials

Every part below is commonly available from Indian and international hobby-electronics suppliers. Prices are indicative 2026 retail figures in Indian rupees and will drift — treat them as a budgeting guide, not a quotation.

Estimated total: ₹16,640, excluding tools, shipping and consumables.

Tools and consumables

• Soldering iron (temperature controlled, 350 °C) with 0.8 mm 60/40 or lead-free solder
• Digital multimeter — continuity, DC volts and current ranges
• Wire strippers, flush cutters and a small set of precision screwdrivers
• Heat-shrink tubing and a heat gun (or a lighter, carefully)
• A laptop with a USB port and the toolchain listed above

Hardware Specifications

Consolidated electrical and interface specifications for every active part in the build.

Power Budget & Supply Sizing

Add up the typical active current of every part, then size the supply with at least 50 % headroom so transmit bursts and motor inrush never brown out the controller.

Summed typical draw is 3468.5 mA. With a 1.5× design margin the supply should deliver at least 5300 mA continuously at the stated rail voltage.

Software Requirements & Development Environment

Reference toolchain: Arduino IDE 2.3.x with the ESP32 board package 3.x (or PlatformIO on VS Code). Anything newer normally works; anything older may lack the board definitions used here.

• Install the Arduino IDE 2.3.x (or PlatformIO if you prefer a real editor and dependency locking).
• Add https://espressif.github.io/arduino-esp32/package_esp32_index.json under File → Preferences → Additional Board Manager URLs, then install esp32 from the Boards Manager.
• Set the correct port under Tools → Port. On Linux add yourself to the dialout group: sudo usermod -aG dialout $USER and log out and back in.
• Open the Serial Monitor at 115200 baud — every sketch here logs its state there.
• Keep File → Preferences → Show verbose output during: compilation switched on while you are debugging build errors.

Required libraries

Block Diagram

The block diagram shows the functional decomposition of the system — what senses, what decides, what acts, and where the data ends up.

Circuit Diagram & Wiring

Every signal line in the build is shown below, followed by a pin-by-pin connection table you can work through with a multimeter in hand.

Wire one row at a time and tick it off — most "it does not work" reports trace back to a single swapped pair.

Wiring explanation

• Mount the probes below the waterline through IP68 glands, with all electronics in a sealed dry compartment above; a single leak drowns the station.
• Put the pH electrode on a proper high-impedance BNC amplifier and keep analogue probe grounds quiet and separate from the modem's current spikes.
• Place the DS18B20 at the same depth as the DO/pH probes so its temperature genuinely compensates them.
• Fit the turbidity sensor's optical faces where a wiper or anti-fouling can keep them clear; a fouled optical face reads ever-rising false turbidity.
• Use an MPPT charger so the panel keeps the pack topped through short winter days and long overcast spells on the water.

System Architecture

Read the stack from the bottom up: physical hardware, the firmware that drives it, the transport that moves data off the device, and the software a human actually looks at.

Working Principle

The buoy's power is temporal, not analytical. A certified lab beats it on accuracy for any single sample, but the lab sees the river once a month; the buoy sees it every few minutes, forever. Rivers change on every timescale — a storm slug of turbid run-off over hours, an illicit discharge over a single night, the daily rise and fall of dissolved oxygen as photosynthesis by day gives way to respiration by night. These are exactly the phenomena grab-sampling is blind to, and exactly what continuous monitoring reveals. The design accepts modest per-reading accuracy to gain the thing that actually catches pollution: presence.

Dissolved oxygen is the river's master health variable, and its behaviour is doubly informative. Its absolute level determines whether fish and invertebrates can live in a reach; its pattern diagnoses the river's metabolism. A healthy stream shows a gentle diurnal DO swing; a river choked with nutrients (eutrophic) shows violent swings — supersaturated by afternoon, crashing to near-zero before dawn — and an organic pollution event drives DO down as microbes consume oxygen breaking down the waste. Because oxygen solubility falls as water warms, DO must always be interpreted together with temperature, and reported both as an absolute (mg/L, for the fish) and as a percentage of saturation (for the metabolism).

pH and turbidity are the two cheapest, most responsive tell-tales of an intrusion. Many industrial and mining discharges shift pH sharply, and biological activity moves it more slowly; a sudden pH step that does not match the diurnal rhythm is a strong discharge signature. Turbidity — how much the water scatters light — tracks suspended sediment and a great many effluents, and because it often changes fast and visibly, a turbidity spike is frequently the very first sign that something has entered the water, even before you know what. Watching the rate of change of these signals, not just fixed thresholds, is what lets the buoy flag an event: a normal river drifts; a discharge steps.

The hard part is not the sensing but the survival. Submerged probes foul — algae and biofilm coat them within weeks, and a fouled optical turbidity face reads a steadily rising false signal while a fouled DO membrane reads progressively low — so the design fights fouling (copper, wipers) and, just as importantly, flags suspected drift so a slow lie is caught. The platform must ride floods and shrug off debris without sinking or dragging its mooring; the electronics must stay dry through constant immersion; and the whole thing must run for months on the sun and log locally so a dropped link never loses the record. A river buoy that reports beautiful data for three weeks and then quietly fouls, floods or floats away has failed at the only thing that mattered — being there when the pollution happened.

The maths behind it

DO percent saturation (temperature-aware)

Report both absolute and relative oxygen:

  DO_pct = DO_meas(mg/L) / DO_sat(T) × 100

DO_sat falls with temperature (~9 mg/L at 20 °C to
~7 mg/L at 30 °C, freshwater). Diurnal DO_pct swinging
from >120% (afternoon) to <40% (pre-dawn) signals a
nutrient-enriched, metabolically stressed river.

Turbidity from scattered light

Optical turbidity sensor: cloudier water scatters more
light to the detector.

  NTU = f(V_scatter)   (calibrated against formazin standards)

Rising baseline over weeks with no rain event = biofouling
of the optical face, not real turbidity → flag & clean.

Event (rate-of-change) detection

For each parameter x, compare to a slow baseline:

  base ← base + α·(x − base)      (α small)
  event if |x − base| > k·σ_recent  sustained N reads

k·σ scales the threshold to each parameter's normal noise.
A step that exceeds the diurnal rhythm's expected range is
flagged as a possible discharge — the cue for a grab sample.

Program Flowchart

The firmware is a single cooperative loop. Nothing blocks for long, so networking, sensing and the user interface all stay responsive.

Assembly Instructions

Build on a breadboard first and only commit to solder once the whole system has run for an hour without a fault.

1. Build the buoy and mooring

   Assemble a sealed, ballasted hull that floats upright with the probes below the waterline and the electronics compartment dry above. Design the mooring for flood flow and debris load — a scope and anchor that will not drag or snap.

   Bring the probe cables into the dry compartment through IP68 glands; pressure-test the seals before deployment.

2. Install and protect the probes

   Fit the DO, pH, turbidity and temperature probes at a representative depth. Apply anti-fouling (copper around optical faces, a wiper if available) and position them where a service visit can reach them.

   Amplify the pH electrode properly and keep analogue grounds away from the modem's switching current.

3. Set up solar, storage and telemetry

   Mount the solar panel on top clear of splash, charge the pack through an MPPT controller, and fit the LoRa (or cellular) modem with its antenna high and dry. Confirm local logging works before you rely on the link.

Step-by-Step Implementation Guide

Work through these in order. Each step ends in something you can observe, so a failure is always localised to the step you just finished.

1. Calibrate every probe against standards

   pH against 4/7/(10) buffers; DO against zero and air-saturated water; turbidity against formazin (or a supplied) standard; EC against a standard solution. Store constants and dates.

2. Compensate and detect events

   Temperature-compensate DO and pH, maintain a slow baseline per parameter, and flag a sustained departure beyond a noise-scaled threshold as a possible discharge event.

   cppriver-events.ino Copy Download

   struct Chan { float base; float var; bool primed; };
   Chan cDO, cpH, cTurb;
   
   // Update a slow baseline and running variance; return true on event.
   bool detect(Chan &c, float x, float k, uint8_t &nOver) {
     if (!c.primed) { c.base = x; c.var = 1; c.primed = true; return false; }
     float d = x - c.base;
     c.var  = 0.98f * c.var + 0.02f * d * d;         // running variance
     c.base += 0.02f * d;                             // slow baseline
     float sigma = sqrtf(c.var) + 1e-3f;
     if (fabsf(d) > k * sigma) { nOver++; }           // step beyond normal noise
     else nOver = 0;
     return nOver >= 3;                               // sustained → event
   }
   
   float doPercentSat(float doMgL, float tempC) {
     // DO_sat approximation (freshwater); replace with a fuller table if needed.
     float sat = 14.6f - 0.41f*tempC + 0.008f*tempC*tempC;
     return doMgL / sat * 100.0f;
   }

   c.var = 0.98f * c.varEach channel tracks its own running variance, so the event threshold adapts to how noisy that parameter naturally is rather than using one fixed number for all.

   if (fabsf(d) > k * sigma)An event is a departure from the slow baseline scaled by the channel's own noise — a step that stands out above the river's normal drift and diurnal wobble.

   return nOver >= 3A single noisy sample does not raise an alarm; the departure must persist for several reads, which distinguishes a real discharge from sensor noise.

   float doPercentSat(Converts absolute oxygen to percent saturation using a temperature-dependent solubility, giving the metabolic view of the river alongside the absolute value the fish care about.

3. Log locally, transmit and sleep

   Write every reading to local storage first, then transmit; on a dropped link, keep logging and forward the backlog on reconnect. Sleep between intervals to make the solar budget.

Complete Source Code

The listing below is complete and compiles as written — there are no elided sections. Read the annotations under each block before you upload it.

/* ═══════════════════════════════════════════════════════════════
   River Water Quality Buoy — ESP32, DO/pH/turbidity/temp, LoRa, solar

   Continuously logs the high-value river signals, temperature-
   compensates DO and pH, detects abrupt changes that signal discharge
   events, and reports over LoRa with local-logging fallback.
   ══════════════════════════════════════════════════════════════════ */

#include <WiFi.h>
#include <OneWire.h>
#include <DallasTemperature.h>
#include <LoRa.h>
#include <SPI.h>
#include <Preferences.h>
#include <math.h>

#define PIN_DO    34
#define PIN_PH    35
#define PIN_TURB  32
#define PIN_EC    33
#define OW_PIN     4
#define LORA_CS    5
#define LORA_RST  14
#define LORA_DIO0  2
#define SLEEP_S  600      // 10 min

OneWire           ow(OW_PIN);
DallasTemperature water(&ow);
Preferences       prefs;

float PH_SLOPE, PH_OFFSET, DO_CAL, TURB_CAL, EC_CAL;

RTC_DATA_ATTR struct { float base, var; bool primed; } cDO, cpH, cTurb;

float avgADC(int pin) {
  long s = 0; for (int i = 0; i < 64; i++) s += analogRead(pin);
  return (s / 64.0f) / 4095.0f * 3.3f;
}

float readpH(float t) {
  float v = avgADC(PIN_PH);
  float tc = (t + 273.15f) / 298.15f;
  return 7.0f + (PH_OFFSET - v) / (PH_SLOPE * tc);
}
float readDO(float t) {
  float v = avgADC(PIN_DO);
  return v * DO_CAL * (1.0f - 0.023f * (t - 20.0f));
}
float readTurb() { return avgADC(PIN_TURB) * TURB_CAL; }   // → NTU
float readEC()   { return avgADC(PIN_EC)   * EC_CAL;   }

float doPercentSat(float doMgL, float t) {
  float sat = 14.6f - 0.41f*t + 0.008f*t*t;
  return doMgL / sat * 100.0f;
}

bool detect(decltype(cDO) &c, float x, float k, uint8_t &nOver) {
  if (!c.primed) { c.base = x; c.var = 1; c.primed = true; return false; }
  float d = x - c.base;
  c.var  = 0.98f * c.var + 0.02f * d * d;
  c.base += 0.02f * d;
  float sigma = sqrtf(c.var) + 1e-3f;
  if (fabsf(d) > k * sigma) nOver++; else nOver = 0;
  return nOver >= 3;
}

void loadCal() {
  prefs.begin("river", true);
  PH_SLOPE  = prefs.getFloat("phS", 0.18f);
  PH_OFFSET = prefs.getFloat("phO", 1.65f);
  DO_CAL    = prefs.getFloat("doC", 3.0f);
  TURB_CAL  = prefs.getFloat("tbC", 1000.0f);
  EC_CAL    = prefs.getFloat("ecC", 1000.0f);
  prefs.end();
}

void transmit(float doMgL, float doPct, float pH, float turb,
              float ec, float t, bool event) {
  LoRa.beginPacket();
  LoRa.printf("{\"buoy\":1,\"do\":%.2f,\"do_pct\":%.0f,\"ph\":%.2f,"
              "\"turb\":%.0f,\"ec\":%.0f,\"t\":%.1f,\"event\":%d}",
              doMgL, doPct, pH, turb, ec, t, event ? 1 : 0);
  LoRa.endPacket();
}

void logLocal(/* to SD/flash */) { /* append timestamped record */ }

void setup() {
  Serial.begin(115200);
  loadCal();
  water.begin();

  water.requestTemperatures();
  float t     = water.getTempCByIndex(0);
  float doMgL = readDO(t);
  float pH    = readpH(t);
  float turb  = readTurb();
  float ec    = readEC();
  float doPct = doPercentSat(doMgL, t);

  static uint8_t nDO, nPH, nTB;
  bool ev = detect(cDO, doMgL, 3.0f, nDO)
          | detect(cpH, pH,    3.0f, nPH)
          | detect(cTurb, turb, 3.0f, nTB);

  logLocal();                              // record first — never lose data

  SPI.begin();
  LoRa.setPins(LORA_CS, LORA_RST, LORA_DIO0);
  LoRa.begin(433E6);
  LoRa.setSpreadingFactor(10);
  transmit(doMgL, doPct, pH, turb, ec, t, ev);

  esp_sleep_enable_timer_wakeup((uint64_t)SLEEP_S * 1000000ULL);
  esp_deep_sleep_start();
}

void loop() {}   // deep sleep restarts setup()

Configuration & Calibration

Configuration steps

• Load probe calibration constants and the event-threshold multiplier (k·σ) per channel; store calibration dates.
• Set the danger-zone limits (e.g. a hard low-DO floor) that alert regardless of rate-of-change.
• Choose the sampling interval (10 min captures diurnal cycles) and the LoRa/cellular telemetry settings.
• Configure local logging and backlog-forwarding for link outages.

Calibration procedure

An uncalibrated sensor produces confident, precise, wrong numbers. Do this once per physical unit and record the constants.

1. pH / DO

   Calibrate pH against buffers and DO against zero and air-saturated water at a known temperature; verify the temperature compensation reproduces the reference across a range.

2. Turbidity

   Calibrate against formazin (or supplied) standards; note the clean-optics baseline so you can later distinguish real turbidity from fouling drift.

3. Fouling baseline

   Record each probe's clean-water reading at deployment; a slow one-directional drift with no rain/event is the fingerprint of biofouling.

Network Architecture & Connectivity

Communication protocol

Readings publish every ~10 minutes; event flags publish immediately. Local logging is authoritative and forwards any backlog on reconnect, so a remote reach with patchy coverage still yields a complete record.

Message contract between the device and the broker.

Cloud platform configuration

A broker feeds a dashboard that trends each reach and, with several buoys, shows a pollutant slug travelling downstream — timing an event between stations to help locate its source.

Dashboard setup

Per-buoy trend panels with event markers and fouling/calibration-age indicators, plus a reach map when multiple buoys are deployed.

Mobile app integration

Immediate alerts on a detected event or a hard danger-zone breach (e.g. DO below the fish-kill floor), prompting a grab sample.

Security considerations

• Sign readings per buoy so enforcement evidence cannot be spoofed.
• Authenticate any configuration/calibration push to the buoy.
• Alert on a buoy going silent — a drowned or vandalised station must be noticed quickly.

Testing Procedure & Expected Output

Test from the bottom up. Confirm power, then each sensor in isolation, then the integrated loop — the first failing step tells you exactly where to look.

Bench-test checklist. If a row fails, stop and fix it before moving on.

Expected output

The dashboard trends DO (absolute and % saturation), pH, turbidity, EC and temperature, marks detected events, and shows each probe's calibration age and fouling flag.

{
  "buoy": 1,
  "do": 4.1,
  "do_pct": 52,
  "ph": 6.3,
  "turb": 210,
  "ec": 640,
  "t": 27.4,
  "event": 1
}

Here a simultaneous DO drop, pH dip and turbidity spike has tripped the event flag — the classic signature of an organic/chemical discharge, and the cue to dispatch someone with a certified sample bottle.

Troubleshooting: Common Errors & Fixes

Performance Optimisation

• Deep-sleep between 10-minute reads; the modem transmit is the main power cost, so keep packets small.
• Average many ADC samples per probe to beat in-water electrical noise.
• Keep adaptive baselines in RTC memory so event detection survives sleep without re-priming.
• Log locally first and batch-forward backlogs rather than blocking on the link.
• Replace every delay() with a millis() comparison — blocking delays are the single most common cause of dropped readings.
• Sample sensors on a fixed cadence and publish on a slower one; you almost never need to transmit at the sampling rate.
• Move networking into its own FreeRTOS task so a slow DNS lookup cannot stall the control loop.
• Use uint8_t / uint16_t where the range allows; on an 8-bit AVR a 32-bit add costs four times as much.
• Batch several samples into one MQTT publish. Radio time, not CPU time, dominates the energy budget.
• Set the MQTT keep-alive to a value that matches your reporting interval so the broker does not churn reconnections.
• For battery builds use deep sleep between samples: an ESP32 drops from ~160 mA awake to about 10 µA asleep, which is the difference between days and months of runtime.

Safety Precautions

• Continuous data indicates when to take a certified grab sample; it is not laboratory evidence on its own.
• Deploy and service buoys with proper water safety — moving water is dangerous; never work alone in the current.
• Secure the lithium pack and seal the electronics against constant immersion.
• Mark the buoy for navigation and moor it so it cannot become a hazard to boats in flood.
• Lithium cells vent and burn when abused. Only use protected cells or a proper BMS, never charge below 0 °C, and never leave a charging pack unattended on a wooden desk.
• Never power an RF module without its antenna fitted — the reflected power destroys the output stage. Check your local licence-free band and duty-cycle limits before transmitting.
• Water and electronics: mount all boards above the maximum possible water line, use drip loops on every cable, and pressure-test plumbing before wiring anything up.
• Wear eye protection when soldering or cutting, and solder in a ventilated space — rosin flux fumes are a respiratory irritant.
• Power the circuit through a bench supply with a current limit while you are testing. A 300 mA limit turns a wiring mistake into a beep instead of a dead board.
• Disconnect power before changing any wiring. Hot-plugging a sensor onto a live bus is the fastest way to lose a controller.

Maintenance

• Clean and recalibrate probes on a schedule; fouling is the dominant long-term error.
• Inspect the mooring and hull seals after every significant flood.
• Replace ageing DO membranes and pH electrodes before they drift out of use.
• Verify local logs and backlog-forwarding, and keep the solar panel clear of splash-fouling.
• Re-check every screw terminal and header after the first week — thermal cycling loosens connections that felt tight on day one.
• Clean the sensing element on a schedule. Optical and electrochemical sensors foul, and a fouled sensor reports plausible nonsense rather than failing outright.
• Log pack voltage. When resting voltage after a full charge drops below about 4.0 V, the cell is near end of life — replace it.
• Wash the panel every few weeks in dusty conditions; a visible dust film costs 15–25 % of the harvest.
• Keep the broker and dashboard containers patched, and rotate device credentials at least once a year.
• Recalibrate at the interval given in the calibration section, and keep the constants in a text file next to the firmware — not only in flash.
• Keep a short logbook of firmware versions and what changed. Six months later you will not remember why that constant is 1.083.

Future Improvements & Upgrades

A working v1 is a platform, not a finish line. These are the upgrades that add the most capability for the least rework.

• Add automated wiper anti-fouling to extend service intervals.
• Deploy several buoys along a reach to triangulate a discharge to a source outfall.
• Add nitrate/ammonium ion-selective sensing for nutrient-pollution specificity.
• Fuse flow and rainfall data to separate storm run-off from illicit discharges automatically.
• Design a proper PCB. Once the breadboard version has run for a month, moving to a two-layer board removes the intermittent-contact failures that dominate prototype faults.
• Add over-the-air firmware updates so you never have to physically reach a deployed node again.
• Add persistent local storage (microSD or the on-chip flash) so a network outage does not create a hole in your data.
• Move configuration out of the source: a captive-portal setup page or a JSON config file makes the build reusable without a recompile.
• Add a battery and solar option so the unit survives a power cut and can be sited away from a socket.
• Write a small test harness that feeds synthetic sensor values through the decision logic, so you can validate thresholds without physically triggering the event.

Frequently Asked Questions

References & Learning Resources

These are the primary sources worth reading in full. Manufacturer datasheets always outrank forum posts when the two disagree.

1. Dissolved oxygen and aquatic life — overviewReference
2. Turbidity — measurement and meaningReference
3. USGS — continuous water-quality monitoringUSGS
4. Eutrophication and diurnal DO swingsReference
5. Biofouling of water-quality sensors — literatureReference