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Science project idea

Constructed wetland model that treats kitchen greywater with reeds and gravel

Many colonies and farms have no sewage treatment for sink and bath water. A bed of gravel planted with reeds can clean greywater cheaply. The project builds a small horizontal-flow bed, feeds it with diluted kitchen greywater and tests the water before and after for turbidity, pH, dissolved oxygen proxy and chemical oxygen demand (COD) estimate.

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What it shows

Many colonies and farms have no sewage treatment for sink and bath water. A bed of gravel planted with reeds can clean greywater cheaply. The project builds a small horizontal-flow bed, feeds it with diluted kitchen greywater and tests the water before and after for turbidity, pH, dissolved oxygen proxy and chemical oxygen demand (COD) estimate.

The science

In a horizontal subsurface flow wetland, water moves slowly through gravel. Solids are trapped, and biofilm on the gravel and plant roots breaks down organic matter using bacteria. Reed roots release some oxygen and provide a surface for microbes. Longer retention time gives better treatment, but the bed can clog if solids are not removed first.

Materials

Cost: under Rs 5,000. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A long plastic tray or cut drum, 100 x 40 x 30 cm
  • Gravel of 10 to 20 mm and coarse sand
  • Reed or canna lily plants from a nursery or nearby pond (use local species)
  • A settling bucket as pre-treatment and a drain tap
  • pH strips, a turbidity tube, a TDS meter and a COD or BOD test kit from a school lab (or send samples to a lab)
  • Kitchen greywater from rinsing utensils with mild soap, diluted
  • Sample bottles

Safety

For this project: Greywater carries germs: wear gloves, wash hands and do not drink or use the treated water. Do not use bleach or strong cleaners in the water. Use the output to water only ornamental plants or discard properly. An adult must supervise.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

11 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Sketch the bed with inlet, gravel zone, outlet and the water level 5 cm below the gravel surface.

  2. Place coarse gravel at both ends, fine gravel in the middle, and plant the reeds into the bed.

  3. Run clean water for 2 weeks so that the plants establish and the biofilm begins to grow.

  4. Prepare a settling bucket where the greywater rests for 6 hours before feeding.

  5. Feed 10 litres per day with a slow drip and record the retention time.

  6. Collect inlet and outlet samples every 3 days.

  7. Measure turbidity, pH, TDS and smell, and if a kit is available, measure COD or ask a laboratory to test a few samples.

  8. Plot the removal percentage against time and look for the day the bed performs best.

  9. Repeat with half the flow rate to double the retention time and compare removal.

  10. Compare the planted bed with an unplanted control bed of the same size.

  11. Write the report with the design, results, area per person needed and the costs compared with a conventional plant.

What you should see

After a few weeks the planted bed reduces turbidity by 70 to 90 percent and organic load by 40 to 80 percent, with longer retention giving better results. The control bed performs less well.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is greywater?

Wastewater from kitchens, bathrooms and laundry that has no toilet waste.

2What do the plants do?

Their roots support microbes, release some oxygen and take up a part of the nutrients.

3What is retention time?

The average time water spends in the bed.

4Why is the treated water still not safe to drink?

It can carry pathogens and dissolved chemicals that need further treatment.

Free ebook: Viva Questions for Science Projects

Take it further

Use two beds in series, a vertical and a horizontal one, and test whether nitrogen removal improves.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

Free ebooks for this project

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