“Water quality” is not a single standard — it is a spectrum matched to end use. Toilet flushing demands almost nothing; drinking water demands the highest level of treatment achievable. The art of water treatment system design is building the minimum treatment train that reliably delivers the right quality to each use, at acceptable cost and maintenance burden.
A treatment train is a series of processes, each removing a specific class of contaminant, applied sequentially from coarsest to finest. Each stage protects the next and progressively improves quality.
Typical treatment train for potable rainwater:
Roof → First-flush diverter → Coarse screen → Sediment filter →
Activated carbon filter → UV steriliser → Potable supply
For non-potable uses (toilet, laundry, irrigation):
Roof → First-flush diverter → Coarse screen → Sediment filter → Tank → Distribution
The key principle: only apply treatment stages that are necessary for the intended use. Each additional stage adds cost, energy, and maintenance.
| Parameter | Unit | Significance | Potable limit (WHO) |
|---|---|---|---|
| Turbidity | NTU | Suspended particles; aesthetic and pathogen indicator | <1 NTU (5 NTU max) |
| pH | — | Acidity/alkalinity; affects pipe corrosion and disinfection | 6.5–8.5 |
| Total dissolved solids (TDS) | mg/L | Dissolved minerals; taste | <600 mg/L |
| E. coli | CFU/100mL | Fecal contamination indicator | 0 (not detectable) |
| Total coliforms | CFU/100mL | Broader contamination indicator | 0 |
| Nitrate (NO₃) | mg/L | Agricultural runoff | <50 mg/L |
| Hardness (CaCO₃) | mg/L | Scale formation in pipes and appliances | <500 mg/L |
| Lead | μg/L | Old plumbing or roof materials | <10 μg/L |
| Zinc | μg/L | Galvanised roof leaching | <3,000 μg/L |
Rainwater characteristics: Freshly collected rainwater is typically near-distilled, with low TDS and near-neutral pH. The main risks are:
The first line of defence. A 100–500 micron mesh screen at the downpipe or tank inlet removes leaves, insects, and large debris.
Remove particles 1–50 microns in size. Installed in-line in the distribution pipe after the tank.
Graded filtration: Use a coarser pre-filter (20 micron) before a finer filter (5 micron) to extend the fine filter’s life.
Remove dissolved organic compounds, chlorine, taste, odour, and some pesticides and herbicides. Effective for improving palatability of potable water.
Not effective for: Pathogens (bacteria, viruses), TDS, heavy metals (except with impregnated carbon), nitrates.
Ultraviolet light at 254 nm wavelength damages DNA in microorganisms, preventing reproduction. UV is highly effective against bacteria, viruses, and protozoa including Cryptosporidium and Giardia (which are resistant to chlorine).
Dose requirement: 30–40 mJ/cm² (millijoules per cm²) for potable water disinfection.
Dose (mJ/cm²) = Intensity (mW/cm²) × Exposure time (s)
For a 40 mJ/cm² dose at 1.0 mW/cm² intensity:
Exposure time = 40 / 1.0 = 40 seconds
UV system sizing by flow rate:
def uv_contact_time(dose_mJ_cm2, intensity_mW_cm2):
"""Required contact time in seconds."""
return dose_mJ_cm2 / intensity_mW_cm2
def uv_chamber_volume(flow_lpm, contact_time_s):
"""Required UV chamber volume in liters."""
flow_lps = flow_lpm / 60
return flow_lps * contact_time_s
t = uv_contact_time(40, 1.0)
V = uv_chamber_volume(20, t)
print(f"Contact time: {t:.0f} s, Chamber volume: {V:.1f} L")
Important limitations:
Chlorine provides both immediate kill of pathogens and residual disinfection protection in the distribution network and tank. Suitable for stored rainwater where re-growth risk exists.
Target residual: 0.2–0.5 mg/L free chlorine in the stored water.
Dosing calculation:
Mass of chlorine (g) = Target concentration (mg/L) × Volume (L) / 1,000,000 × (1 / purity fraction)
For a 10,000 L tank, target 0.3 mg/L residual, using 5% sodium hypochlorite solution:
Mass of hypochlorite solution (mL) = 0.3 × 10,000 / (1,000 × 0.05 × 1000) × 1,000,000
= 0.3 × 10,000 / 50 = 60 mL
Or more simply: Volume (mL) of 5% bleach = Target (mg/L) × Tank volume (L) / 500
= 0.3 × 10,000 / 500 = 60 mL
Chlorine limitations: Does not kill Cryptosporidium or Giardia. Reacts with organic matter to form trihalomethanes (THMs) — monitor total organic carbon in tank water.
RO membranes remove almost everything: bacteria, viruses, TDS, heavy metals, nitrates, and most dissolved organics. Output water is nearly pure.
RO is energy-intensive (~50–150 Wh per m³) and not typically needed for rainwater that has been adequately filtered and disinfected. Most appropriate where source water has specific chemical contamination issues.
| End use | Required quality tier | Minimum treatment |
|---|---|---|
| Drinking / cooking | Potable (Tier 1) | Sediment + Carbon + UV (or chlorine) |
| Tooth-brushing | Potable (Tier 1) | Same as above |
| Showering / bathing | Hygienic non-potable (Tier 2) | Sediment filter; UV optional |
| Hand-washing | Tier 2 | Sediment filter |
| Toilet flushing | Non-potable (Tier 3) | Coarse screen; sediment filter |
| Washing machine | Tier 3 | Sediment filter |
| Garden irrigation (sprinkler) | Tier 3–4 | Sediment filter; no spray near edible plants |
| Garden irrigation (subsurface drip) | Tier 4 | Minimal; sediment filter |
Filter cartridge service life depends on flow rate and incoming water quality (turbidity load).
Service life estimation:
Service life (days) = Filter capacity (L) / Average daily flow (L/day)
A 5-micron cartridge rated for 20,000 L of water at 10 NTU turbidity, with a household using 600 L/day through the filter:
Service life = 20,000 / 600 = 33 days
In practice, most cartridges are replaced every 1–3 months on a schedule, not based on calculated throughput.
Sand filter hydraulic loading rate: For slow sand filters used in larger systems:
Hydraulic loading rate = Flow rate (m³/hour) / Filter area (m²) = 0.1–0.4 m³/m²/hour
A filter treating 1 m³/hour at 0.2 m³/m²/hour loading rate needs a filter bed of 5 m².
Before commissioning a system for potable use, and periodically thereafter, water must be tested.
Commissioning test minimum:
Laboratory testing: Use an accredited laboratory. A standard potable water test panel costs €50–€150 and covers the key parameters. Specify “rainwater for domestic potable use” to get the appropriate test panel.
DIY test kits: Available for chlorine residual, pH, TDS, and basic coliform tests (dip-strip type). Useful for routine monitoring but not a substitute for laboratory analysis for initial commissioning.
Previous: Chapter 5 — Pipes, Pumps, and Infrastructure
Next: Chapter 7 — Water Once More: Greywater Recycling Systems