Turbidostat: Open-Source Biomass Density Control for Microalgae

Turbidostat system overview

This system was developed at the Biodesign Swette Center for Environmental Biotechnology, Arizona State University. The scientific contribution was published in Algal Research (2018). Total hardware cost: ~$200 (excluding the lab-grade peristaltic pump).

What is a Turbidostat?

A turbidostat maintains microalgae culture at a constant density by continuously monitoring turbidity (optical density) and diluting with fresh medium when the culture grows too dense. Think of it as a thermostat, but for biomass concentration instead of temperature.

Commercial turbidostat systems cost $10,000+. This open-source build achieves the same function for ~$200 using a $15 turbidity sensor and an Arduino.

The Problem

Measuring microalgae biomass typically requires a laboratory spectrophotometer — expensive, manual, and limited to one sample at a time. For continuous cultivation experiments (days to weeks), an automated, low-cost system that monitors and controls density in real-time is essential.

The target organism was Synechocystis sp. PCC 6803, a sub-micron cyanobacterium that converts light energy and CO2 into biomass. The challenge: finding an affordable sensor that works with these very small cells.

Design

Concept

Design concept Core concept: turbidity sensor reads culture density → Arduino compares to setpoint → activates dilution pump when density exceeds threshold.

Key Components

Part	Purpose	Cost
TSD-10 turbidity sensor (Amphenol)	Infrared light transmission through culture	~$15
Arduino Mega	Sensor reading, control logic, pump switching	~$20
Op-amp breakout	Amplify sensor signal	~$5
Precision potentiometer (10-turn)	Set target density	~$5
OLED display (SSD1306)	Real-time readout	~$5
2-channel relay board	Switch pumps on/off	~$5
Peristaltic pump (12V)	Dilution delivery	~$30
Yun Shield OR Raspberry Pi	Timestamp + data logging	~$35
Three-position switches (x2)	Manual override control	~$5

Schematic

Circuit schematic Wiring: turbidity sensor → op-amp → Arduino ADC. Arduino drives relay board controlling pumps. Potentiometer sets density threshold.

Control Logic

Flowchart Execution sequence: count interval → flush sampling line → read sensor → compare to setpoint → pump on/off → log data.

Sensor Performance

Sensor internal structure TSD-10 uses infrared LED emission and a photodetector. Designed for washing machines, but works well for microalgae in the OD 0.1-2.0 range.

Calibration: voltage to optical density Conversion of raw voltage readout to optical density (OD730 and NTU). Higher voltage = less light absorbed = lower density.

Turbidostat in action Working system: culture density (green) tracks the setpoint (red). Pump status shows dilution events triggered when density exceeds threshold.

Data Logging

Two options were implemented:

Option 1: Yun Shield (Linux on Arduino)

The Yun Shield provides WiFi, USB, and SD card storage with a Linux timestamp. Data logged to microSD in CSV format.

Control box with Yun Shield Version 1.0: Yun Shield + Arduino Mega + op-amp in a project box.

Raw data from SD card CSV data: timestamp, turbidity reading, setpoint, pump status.

Option 2: Raspberry Pi

Arduino sends Serial.print() data over USB; a Python script on the Pi captures, timestamps, and saves to file.

RPi logging concept Simplified data path: Arduino → USB serial → Python script → timestamped CSV.

// Arduino sends this every 60 seconds:
String dataString = "";
averageRead = round((read25+read35+read45+read55)/4);
dataString += String(averageRead) + "," + String(setValue) + "," + String(_pumpON);
Serial.println(dataString);

Hardware Photos


Relay board for pump switching	Control box (Version 1.0)


Turbidity sensor sampling from reactor	Sensor wrapped in foil to block ambient light


Complete photobioreactor with turbidostat	Harvested microalgae from hold-up tank

Publication

This turbidostat is one of three open-hardware units developed for an advanced photobioreactor system. The scientific contribution — demonstrating that a $200 system can replace $10,000+ commercial alternatives for continuous microalgae cultivation — was published in:

Algal Research (2018) DOI: 10.1016/j.algal.2018.03.013

Publication screenshot

The paper covers:

Validation of low-cost turbidity sensing against laboratory spectrophotometer
Long-term continuous cultivation results
Open hardware design enabling reproducibility
Cost comparison with commercial systems

Source Code

Arduino (Yun Shield): Turbidity_logdat_YunShield_2018
Arduino (RPi logging): Turbidity_log_python
Python data capture: serial_logger.py

Key Takeaways

A $15 washing-machine turbidity sensor works for microalgae optical density measurement
Total system cost ~$200 vs $10,000+ for commercial alternatives
Arduino + relay + peristaltic pump = automated density control
Two data logging options: embedded Linux (Yun Shield) or Raspberry Pi with Python
Published validation in peer-reviewed journal confirms scientific utility

Developed at Biodesign Swette Center for Environmental Biotechnology, Arizona State University. Published 2018.