Anything in your water can affect how well a treatment system works — whether that system is chemical or physical. You don't need a chemistry degree to design good treatment, but there are nine basic factors worth understanding before choosing equipment: pH, hardness, iron, manganese, gases, turbidity, dissolved solids, tannins, and UV transmittance.
pH
pH measures the concentration of hydrogen ions in water on a scale from 0 to 14. Lower is more acidic, higher is more basic, and 7 is neutral. Most drinking water sits in the 7–8 range. Each whole-number change is a tenfold change in pH, so jumping from 8 to 9 is a big shift and worth investigating.
For UV systems specifically, water with a pH below 6 can begin to pit the stainless steel reactor and eventually cause leaks. pH also matters when designing softening or tannin removal.
Hardness
As rain falls it absorbs carbon dioxide and becomes slightly acidic. As that slightly acidic water moves through the ground, it dissolves calcium and magnesium bicarbonates from limestone and similar minerals. When the resulting "hard" water is heated — in a water heater or even just sitting in a UV reactor — those minerals fall out of solution and form white scale on every surface they touch, including the quartz sleeve of a UV lamp. That scale blocks UV light and stops disinfection from working properly.
UV manufacturers recommend removing hardness above 7 gpg (about 120 mg/L) — usually with a water softener.
Iron & Manganese
Iron and manganese dissolve into groundwater the same way hardness does. When the water is exposed to oxygen at the surface, they precipitate out and form scale — orange from iron, blackish-grey from manganese. Both will discolor a quartz sleeve and block UV light. Recommended pre-treatment thresholds:
- Iron: above 0.3 mg/L
- Manganese: above 0.05 mg/L
Dissolved Gases
All water contains some dissolved gas. At elevated concentrations, gases can cause sensor issues in UV systems. Hydrogen sulfide is a common offender — it produces a rotten-egg smell and at higher levels can stain fixtures and the quartz sleeve. Gas concentrations can change throughout the year as barometric pressure shifts.
Turbidity
Turbidity is the haze caused by tiny suspended particles in water. Those particles interfere with both chemical disinfectants and physical disinfection like UV — a single speck of turbidity can shield bacteria from UV light. That's why a properly sized sediment filter (typically a 5-micron pre-filter) is essential ahead of any UV system.
Tannins
Tannins are natural organic compounds — fulvic and humic acids — that form as water passes through decaying vegetation or peaty soil. They give water a faint yellow, "tea-like" color and can produce earthy or musty odors and staining. Tannins are an aesthetic problem in drinking water, but they can absorb UV light and dramatically reduce the effectiveness of a UV system. Levels should be below 0.1 mg/L for reliable UV performance. Removal usually involves an organic-scavenging anion exchange resin, often paired with a softener for protection.
UV Transmittance (UVT)
UVT measures how well a fluid transmits UV light. Municipal water supplies typically have UVT levels above 95%; deep wells are often around 85%. Surface water varies widely. Most residential UV systems are rated at a 95% UVT baseline. If your water has lower UVT, either the delivered dose drops or the flow rate has to be reduced to keep the dose where it should be. As a rule of thumb, water treated with UV should have a UVT of at least 75%.