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2.5 Biochemical oxygen demand: a key indicator of water quality
Biochemical oxygen demand is the amount of dissolved oxygen needed by aerobic organisms to break down organic material present in a water sample. It is a common indicator of organic biodegradable pollution and used to assess the effectiveness of wastewater treatment plants (Clair et al., 2003). Concentrations of BOD normally increase as a result of organic pollution caused by discharges from wastewater treatment plants, as well as industrial effluents and agricultural run-off. Severe organic pollution may lead to rapid de-oxygenation of river water, and can harm fish and aquatic invertebrates. BOD concentrations therefore provide an insight into the quality of water for aquatic life.
The most important sources of organic waste are from domestic wastewaters, from certain industries such as paper or food processing, and from agriculture in silage effluents and manure.
As outlined in Annex A3, the UWWTD regulates waste water discharges from domestic sources, certain industries such as paper or food processing, and from agriculture in silage effluents and manure. The directive explicitly specifies which kind of treatment must be applied. Biological wastewater treatment (‘secondary treatment’) significantly reduces biodegradable pollution in wastewater and therefore has a direct influence on the surface water quality, in particular the organic carbon expressed as BOD.
2.5.2 Reduction of BOD in rivers due to urban wastewater treatment
Industrial and agricultural production increased in most European countries after the 1940s. Owing to a greater share of the population being connected to sewage collection and treatment systems, the discharge of organic waste into surface water increased. Over the past 15 to 30 years, however, more and waste water treatment plants have introduced biological treatment (secondary treatment) and organic discharges have subsequently decreased throughout Europe.
In European rivers, biological oxygen demand (BOD) decreased by 1.6 mg/l between 1992 and 2012 (Figure 2.6).
Figure 2.6 Trend of 5-day BOD in European rivers.[1]
Figure 4.1 depicts two time series: the longer time series has fewer stations (539) and the shorter time series has more stations (1 235).
1992-2012:, Austria, Belgium, Bulgaria, Denmark, Estonia, Finland, France, Ireland, Latvia, Lithuania, Luxembourg, former Yugoslav Republic of Macedonia, Slovakia, Slovenia, United Kingdom.
2000-2012: Austria, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Denmark, Estonia, Finland, France, Ireland, Italy, Latvia, Lithuania, Luxembourg, Former Yugoslav Republic of Macedonia, Poland, Romania, Slovakia, Slovenia, United Kingdom.
[1] BOD can be measured over 5 days, or less commonly, over 7 days: in figure 2.6, BOD7 data have been recalculated into BOD5 data.
Figure 2.7 charts the changes of BOD concentration in rivers from 2000 to 2012 and the related percentages of the population connected to at least secondary treatment of municipal wastewater in 2010. There is a clear relation between BOD concentration in 2012 (red squares) and the percentage of inhabitants with secondary or more stringent treatment. However, Italy (94 % of inhabitants with adequate treatment), Belgium (73 %) and Poland (66 %) have BOD concentrations above 2 mg/l, which can be regarded as the cut-off point between slightly and moderately polluted rivers (e.g. LAWA 1998).
The largest decreases of BOD concentration between 2000 and 2012 were in Macedonia, Luxembourg, Bulgaria and France. Concentrations fell by 54 %. Almost all countries with low concentrations of BOD (except Ireland, Latvia and Slovenia) have high proportions of the population (above 80 %) with at least secondary treatment. In all countries with high percentages of the population connected to at least treatment, BOD concentrations in the river are low or the water quality is gradually improving.
Figure 2.7 Changes of BOD concentrations in rivers between 2000 and 2012, and proportion of population with at least secondary treatment (2010).
Note: Percentages in parentheses are inhabitants with secondary or more stringent treatment of urban wastewater (2010).
Sources: http://www.eea.europa.eu/data-and-maps/indicators/freshwater-quality/freshwater-quality-assessment-published-may-2; http://ec.europa.eu/eurostat/tgm/table.do?tab=table&init=1&language=en&pcode=ten00020&plugin=1
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POLAND
The diagram shows BOD5 for Poland 3,5-4 both for 2000 and 2012. How was it calculated? The Average from BOD in rivers (annual average) reported for the year 2000 is 4,35 (http://cdr.eionet.europa.eu/pl/eea/ewn1/env1033029122/) and for 2012 - 2,8 (http://cdr.eionet.europa.eu/pl/eea/ewn1/envunzbxa/).
Addressed.
ROMANIA
Please reformulate:
As outlined in Annex A3, the UWWTD regulates waste water discharges from domestic sources, certain industries such as
paper orfood processing, and from agriculture in silage effluents and manure.In Annex III Indusrial sectors of the UWWTD there are not included the paper industry, only 11 type of food industry
Addressed.
AUSTRIA
In the report the term ‘wastewater’ and the term ‘waste water’ is found. A consistent wording should be used
Chapter 2.5.2, line "Over the past 15 to 30 years, however, more and waste water....": Phrasing is not clear
Addressed.