Summary
A 200-500L modular household rainwater collection cistern with an integrated low-cost optical turbidity sensor that automatically diverts the first flush and high-turbidity water to garden irrigation via a solenoid valve, while routing cleaner water to a potable storage tank after basic filtration. The system uses a microcontroller to monitor water clarity in real-time and provides visual indicators for water quality status.
Target user
Suburban homeowners in regions with seasonal rainfall (annual precipitation >600mm) who want to supplement municipal water supply for drinking and reduce utility costs, particularly in areas with water scarcity concerns or high water prices ($3+ per 1000L).
Functional mechanism
Rainwater enters the cistern through a debris screen and first-flush diverter that automatically discards the initial 10L per rainfall event (containing roof contaminants). The turbidity sensor continuously monitors water clarity by measuring light scattering at 90 degrees using an infrared LED and photodiode positioned in a flow-through chamber on the inlet pipe. When turbidity exceeds 25 NTU (threshold based on WHO guidelines suggesting <5 NTU for treated drinking water, with margin for home filtration), the microcontroller triggers the solenoid valve to divert flow to a gray-water outlet for garden use. Water below the threshold passes through the activated carbon filter into the potable storage section. The system assumes users will perform additional point-of-use treatment (boiling or UV sterilization) as turbidity sensors cannot detect microbiological contamination, only suspended particles.
Estimated cost
$280-$420 per unit
Materials
- Food-grade HDPE cistern tank (200-500L capacity)
- Stainless steel mesh pre-filter (1mm aperture)
- Low-cost turbidity sensor (TS-300B or DFRobot SEN0189, 0-3000 NTU range)
- Arduino Nano or ESP8266 microcontroller
- 12V DC solenoid valve (normally-open, 3/4 inch NPT)
- Solar panel (10W) with charge controller and 12V 7Ah lead-acid battery
- PVC piping (3/4 inch, Schedule 40)
- LED indicator lights (green/yellow/red for quality status)
- Waterproof enclosure (IP65 rated) for electronics
- Activated carbon block filter (5 micron, replaceable cartridge)
Manufacturing
- Injection mold or rotationally mold food-grade HDPE tank with integrated baffle separating potable (60%) and gray-water (40%) compartments, including molded NPT threaded ports for inlet, outlets, and sensor housing
- Machine stainless steel sensor flow-through chamber with optical windows, mount TS-300B sensor module with silicone gasket seal
- Assemble control circuit: flash Arduino Nano with turbidity threshold logic (25 NTU trigger, 15 NTU reset for hysteresis), wire to sensor analog output, relay module for solenoid, voltage regulator, and status LEDs
- Install solar charge controller circuit in waterproof enclosure, connect 10W panel, 12V battery, and wire to main control board with inline fuse protection
- Cut and solvent-weld PVC piping manifold: inlet with first-flush diverter (ball-float shutoff design), sensor chamber inline, solenoid valve splitting to potable filter pathway and gray-water outlet
- Mount activated carbon filter housing on potable outlet line with quick-connect fittings for cartridge replacement
- Pressure test entire system at 30 PSI for 10 minutes to verify no leaks at joints or sensor chamber seals
- Calibrate turbidity sensor using formazin standards (0, 20, 50, 100 NTU) and adjust microcontroller threshold values, document calibration curve
- Apply UV-resistant coating to exterior surfaces, affix installation instructions and water quality disclaimer labels
Risks
- Turbidity sensors cannot detect dissolved contaminants, heavy metals, pesticides, or pathogens - users may falsely assume water safety based solely on clarity, requiring prominent warnings and education about supplementary treatment needs
- Sensor fouling from biofilm growth or mineral deposits will cause false readings within 3-6 months without regular cleaning protocols, potentially allowing contaminated water through or wasting clean water
- Regional variation in roof materials and atmospheric pollution creates inconsistent turbidity baselines (urban areas with high particulate pollution may never achieve <25 NTU), requiring user-adjustable thresholds that complicate the 'low-cost' value proposition
- Solar power system may be insufficient during extended cloudy periods (>5 days), requiring battery oversizing or grid backup that increases costs by $60-$100
- Regulatory barriers in many jurisdictions prohibit rainwater for potable use without certified treatment systems, limiting market to permissive regions or non-potable applications only
- First-flush diverter requires manual reset after each rain event in low-cost ball-float designs, reducing automation benefit and requiring user discipline
Sustainability claims
- Reduces household municipal water consumption by 30-80L per day (assuming 2-4 person household using rainwater for drinking and cooking), decreasing energy for water treatment and pumping infrastructure at an estimated 0.4 kWh saved per cubic meter
- Diverts 2000-8000L annually from stormwater runoff (depending on roof area and precipitation), reducing urban flooding stress and combined sewer overflow events in dense areas
- Eliminates need for 1000-2000 single-use plastic water bottles per household annually in regions where tap water distrust drives bottled water consumption
- Food-grade HDPE cistern is recyclable at end-of-life (15-20 year lifespan), with material recovery rates of 30-40% in established HDPE recycling streams
- Solar-powered operation creates zero operational carbon emissions, with embedded energy of system (estimated 180 kWh for manufacturing) offset within 2-3 years through avoided grid-supplied water pumping
- Promotes water literacy and conservation behavior change by making rainfall harvesting visible and quantified for households
Next experiment
Build a benchtop prototype of the turbidity sensor assembly and microcontroller logic using a TS-300B sensor, Arduino Nano, and transparent acrylic flow chamber, then test response curves with controlled contamination scenarios (garden soil suspension, leaf debris tea, clay particles at 5/10/25/50 NTU concentrations) to validate threshold accuracy and determine realistic cleaning interval requirements before scaling to full cistern integration.