ESTONIAN ENVIRONMENTAL REVIEW 2009

6.3. Water pollution load
To plan water treatment equipment and determine
the level of water contamination risk, the pollution load
must be known – i.e. the amount of waste substances that
constitutes the load on the treatment facility, water body
or environment. The primary indicators used to evaluate
the water released from the treatment facility into the
water bodies are biochemical oxygen demand (BOD 7
) F ,
suspended solids, phosphorus and nitrogen content. Pollution
load is expressed in population equivalent – a unit
that stands for the average water pollution load generated
by one person in a 24-hour period. The population
equivalent value of biochemical oxygen demand (BOD 7
)
is 60 g of oxygen per day.
Starting from 1992, the pollution load in Estonia has
decreased significantly (figure 6.7). Compared to 1992,
the annual BOD 7
load has decreased 94%, while the drop
in phosphorus and nitrogen in the same period has been
79% and 71%, respectively. The most important reason
for the drop in the load is the decrease in industrial and
agricultural production starting in the beginning of the
1990s. In recent years, the drop in the load had accelerated
thanks to significant investments as a result of which a
number of wastewater treatment plants have been built
and renovated. One reason for reduced pollution load is
also the rise in pollution taxes and stricter requirements
for wastewater treatment. By 2007 the decline in pollutants
in wastewater had halted and remained on the
level of 2006.
An average of 85–95% of pollutants are removed from
wastewater during wastewater treatment. In 2007, 99%
of all water needing to be treated was treated.
The effectiveness of treatment has grown rapidly
in recent years. Removal of phosphorus and nitrogen
(tertiary treatment) is today one of the primary goals of
wastewater treatment, and as a result the share of biological
chemical treatment G has risen (figure 6.8). Currently
around 78% of all residential and industrial wastewater
that needs treatment undergoes tertiary treatment.
Whereas only 63% of treated water was sufficiently pure
in 1992, the figure had risen to 98.7% as of 2007. Forty
per cent of wastewater that requires treatment is generated
in Tallinn. Before being returned to water bodies,
mining water first undergoes partial treatment (primarily
removal of suspended particles) in settling basins.
Mining waters do not have a significant impact on the
composition of natural waters; only sulphate concentrations
increase significantly. Most Estonian wastewater is
cooling water, which does not require treatment. To this
point, the lowest amount of wastewater was generated
in the extraordinary dry year of 2006.
Directive 91/271/EEC of the Council of the European
Union governs wastewater treatment in over settlements
with a more than 2000 population equivalent. Around
900,000 people live in such areas – about 69% of Estonia’s
population. In 2007, 92% of these people were furnished
with sewerage connections and 94% of them used public
water works.
In 2007, the volume of wastewater and total nitrogen
increased while BOD 7
fell. The BOD 7
indicator dropped
by 25% over the year, primarily on account of the city
of Kehra. The amount of nitrogen increased by 5% on
account of Kohtla-Järve and Tallinn. The trend is the same
with regard to total phosphorus. The pollution level in
the water routed to Tallinn treatment plant has risen in
recent years in connection with city sewerage and connection
of new areas to the system. Nitrogen removal was
launched in Tallinn in mid-2005; the level of 10 mgN/l for
water discharged into the environment, established in the
urban wastewater directive, has not yet been achieved
(the average for 2007 was 10.5 mgN/l).
Renovation of the Kohtla-Järve treatment plant was in
progress in 2007; the treatment capacity was reduced and
therefore the goals of 70% nitrogen and 80% phosphorus
reduction were not achieved. The treatment plant is now
completed and in operation.
Starting in 2003, wastewater treatment plants were
renovated in Tallinn, Kohtla-Järve, Narva, Tartu, Valga,
Põltsamaa, Otepää, Kohila and Jüri. The Märjamaa treatment
plant is being renovated. By the end of 2004, in
connection with the completion of the Kösti treatment
plant, Viljandi’s untreated wastewater is no longer
released directly into the environment. In 2007, the new
wastewater treatment plant in Paldiski was launched,
where emissions were previously treated mechanically
(primary purification). The treatment efficiency
of some treatment plants is still not sufficient to bring
wastewater into conformity to the requirements. Türi
and Tõrva’s wastewater treatment plants still must be
renovated – currently wastewater is treated in bioponds,
the capacities of which are several times below
the increasing load.
F
Biochemical oxygen demand BOD7 indicates the amount of oxygen necessary for biological decomposition of (an)organic matter contained in water over 7 days.
G
Biological chemical treatment – besides conventional biological treatment, nitrogen and/or phosphorus is also removed from wastewater.
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