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3.1.5. Stability in water
The methodological guidelines on setting hygienic norms for water bodies which are
used for extracting drinking, bathing and household water (МU 2.1.5.720-98) involve the
investigation of the stability of a substance and its degradation in water. Although their
primary purpose is the justification of the hygienic MPC, the results can serve as a basis
for choosing the most suitable wastewater treatment method.
The stability/degradation investigation involves the following steps:
>> Theoretical assessment, taking into account the physico-chemical properties, the
technological documentation and the material safety data sheets regarding storage
conditions and time and recommended methods for wastewater treatment.
>> Preliminary qualitative assessment of biodegradability in natural self-purification
processes, or in biological wastewater treatment facilities based on the dynamics
of the biological oxygen demand (BOD) measurements.
>> If the dynamics of the BOD measurements reveal that the substance may degrade,
further investigations in a modelled water body are performed. These investigations
involve aerobic biodegradation tests (performed in standardised conditions
according to GOST R 50595-3).
>> Degradation of the substance is tested with destruction factors normally applied in
water treatment processes (chlorine, ozone- and UV-treatment) and food processing
(boiling). Stability and degradation is evaluated by applying organoleptic, physico-chemical
analytical and biological methods.
Substances are classified according to stability to 4 classes – see Table 2. Τ 1/2
is halfperiod
of degradation.
Table 2:
Stability classes of substances in hygienic assessment
Stability class Τ 1/2
of natural
self-purification
Destruction
in UV, heat,
etc treatment
Biodegradability (active
sludge)
Induction
time of active
sludge
Τ 1/2
of degradation
Relevance to
the results of
hygienic assessment
1. Extremely stable > 15 days < 40% > 25 days > 4.33 hours MPC has safety
factor 10 and
hazard class
1 unit stricter
2. Stable 1-15 days 40-80% 3-25 days 1.5-4.33 hours MCP has safety
factor 3
3. Less stable 1-24 hours 81-95% 2-10 days 0.22-1.5 hours Degradation
4. Unstable < 60 minutes > 95% < 3 days < 0.22 hours
products are
also given MCP
26 | The Russian System of Chemicals Management