Activated Sludge

// Dissolved-oxygen + temperature logger for a bench activated-sludge reactor.
// Board: ESP32 DevKit v1.  Probe: analogue galvanic DO probe.  Temp: DS18B20.
//
// WHY TEMPERATURE IS NOT OPTIONAL: the oxygen a given water CAN hold falls by roughly
// 2% per degree C. An uncompensated mg/L reading drifts with the room and is not a
// measurement. This logs raw mV, temperature, mg/L and percent saturation so a bad
// calibration can be spotted after the fact instead of quietly poisoning the data.

#include <OneWire.h>
#include <DallasTemperature.h>
#include <SPI.h>
#include <SD.h>

#define DO_PIN          34      // ADC1_CH6 - ADC2 is unusable while WiFi is on
#define ONE_WIRE_PIN     4
#define SD_CS_PIN        5
#define LOG_INTERVAL_MS 30000UL

// --- calibration, REPLACE WITH YOUR OWN -------------------------------------------
// Zero:       probe in fresh sodium sulfite solution (strips O2 completely).
// Saturation: probe in water aerated hard for 20 min, note the temperature.
const float CAL_ZERO_MV    = 0.0f;
const float CAL_SAT_MV     = 1180.0f;
const float CAL_SAT_TEMP_C = 20.0f;

// Saturated DO in fresh water at 1 atm, mg/L, 0..40 C in 1 C steps.
// Standard table - do not "simplify" to a linear fit; the curve is not linear.
const float DO_SAT_TABLE[41] = {
  14.62,14.22,13.83,13.46,13.11,12.77,12.45,12.14,11.84,11.56,
  11.29,11.03,10.78,10.54,10.31,10.08, 9.87, 9.67, 9.47, 9.28,
   9.09, 8.92, 8.74, 8.58, 8.42, 8.26, 8.11, 7.97, 7.83, 7.69,
   7.56, 7.43, 7.30, 7.18, 7.07, 6.95, 6.84, 6.73, 6.63, 6.53, 6.41
};

OneWire oneWire(ONE_WIRE_PIN);
DallasTemperature sensors(&oneWire);
unsigned long lastLog = 0;
bool sdReady = false;

float doSaturation(float tempC) {
  if (tempC <= 0)  return DO_SAT_TABLE[0];
  if (tempC >= 40) return DO_SAT_TABLE[40];
  int i = (int)tempC;
  float frac = tempC - i;
  return DO_SAT_TABLE[i] + frac * (DO_SAT_TABLE[i + 1] - DO_SAT_TABLE[i]);
}

float readProbeMv() {
  // 64 samples: the reactor is electrically noisy - a feed pump, an air pump, a heater.
  uint32_t acc = 0;
  for (int i = 0; i < 64; i++) { acc += analogRead(DO_PIN); delay(2); }
  return (acc / 64.0f) * (3300.0f / 4095.0f);   // mV, 3.3 V reference
}

void setup() {
  Serial.begin(115200);
  analogSetPinAttenuation(DO_PIN, ADC_11db);    // full 0-3.3 V range
  sensors.begin();

  sdReady = SD.begin(SD_CS_PIN);
  if (sdReady) {
    if (!SD.exists("/do_log.csv")) {
      File f = SD.open("/do_log.csv", FILE_WRITE);
      if (f) { f.println("millis,temp_c,probe_mv,do_mg_l,do_pct_sat"); f.close(); }
    }
  } else {
    Serial.println("SD card not found - serial output only");
  }
  Serial.println("millis,temp_c,probe_mv,do_mg_l,do_pct_sat");
}

void loop() {
  if (millis() - lastLog < LOG_INTERVAL_MS) return;
  lastLog = millis();

  sensors.requestTemperatures();
  float tempC = sensors.getTempCByIndex(0);
  if (tempC < -100) { Serial.println("# temp sensor not responding"); return; }

  float mv = readProbeMv();

  // The probe's saturation output itself drifts with temperature, so scale the
  // calibration point to the temperature we are actually reading at.
  float satMvNow = CAL_SAT_MV * (doSaturation(tempC) / doSaturation(CAL_SAT_TEMP_C));
  float pctSat   = (mv - CAL_ZERO_MV) / (satMvNow - CAL_ZERO_MV) * 100.0f;
  if (pctSat < 0) pctSat = 0;
  float doMgL    = pctSat / 100.0f * doSaturation(tempC);

  char line[96];
  snprintf(line, sizeof(line), "%lu,%.2f,%.1f,%.2f,%.1f",
           millis(), tempC, mv, doMgL, pctSat);
  Serial.println(line);

  if (sdReady) {
    File f = SD.open("/do_log.csv", FILE_APPEND);
    if (f) { f.println(line); f.close(); }
  }

  // Operating window: below 0.5 mg/L filaments outcompete floc-formers and the
  // sludge starts to bulk. Above 3 mg/L you are paying for air you do not need.
  if (doMgL < 0.5f)      Serial.println("# WARNING low DO - bulking risk");
  else if (doMgL > 3.0f) Serial.println("# note: over-aerated, throttle the pump");
}