2. Protecting surface waters from nutrient and organic pollution

2.1 Introduction

For water industry directives, cross-cutting interests are predominantly around limiting pollutants entering water sources or watercourses. Excess nutrients, from wastewater treatment plants and agriculture, are addressed by the UWWTD and, indirectly, by the DWD. Organic pollution can be measured by the biochemical oxygen demand (BOD), which is a key determinant under the UWWTD.

Challenges to maintaining and improving water quality remain, for example diffuse pollution from agriculture as well as point source pollution from sewer overflows.  

2.2 Pressures and status of nutrient pollution

Excess nutrients can cause eutrophication, a process characterised by increased plant growth, proliferation of algal blooms and an undesirable disturbance to the species composition and abundance of the organisms in the water (EEA, 2012).

In addition, eutrophication can lead to more serious problems, such as low levels of oxygen dissolved in the water. This happens when large amounts of algae die and decay. The process of decay uses up the oxygen in the water. Like the effects of high loads of organic matter in wastewater (high BOD, described in chapter 3), this can kill fish and other aquatic organisms. In rivers and lakes, eutrophication is mainly caused by high concentrations of phosphorus, whereas nitrogen enrichment is the main reason for eutrophication in coastal and marine waters (Chislock et al. 2013).

Nutrients in groundwater could also be a source of pollution for surface waters, if rivers or lakes are strongly influenced by groundwater, e.g. where groundwater levels are particularly high in the lowlands. Groundwater or surface water with high nitrate concentrations can pose a risk to human health.

The main sources of nitrogen and phosphorus are point source emissions from UWWTPs and industry, and diffuse emissions from agriculture such as from fertilisers and manure.

Figure 2.1 shows a decline in the nitrogen and phosphorus emissions from the domestic sector in 1990, 2000 and 2009.

Figure 2.1 Intensity of nitrogen and phosphorus emissions from the domestic sector

Data source: http://www.eea.europa.eu/data-and-maps/indicators/emission-intensity-of-domestic-sector/assessment. The nutrient emission intensity here is expressed in kilograms of nutrient discharged into the environment per inhabitant per year (kg/inhabitant x year).

Figure 2.2 shows a decline in the nutrient emission intensity of the manufacturing industry in Europe during 2004, 2007, 2010 and 2012. (Further information is available at http://www.eea.europa.eu/data-and-maps/indicators/emission-intensity-of-manufacturing-industries-1/assessment.) Note that interpretation of this graph may be biased due to the influence of the economic crisis in 2008.

Figure 2.2 Intensity of nutrient emissions from the manufacturing industry.

Data source: http://www.eea.europa.eu/data-and-maps/indicators/emission-intensity-of-manufacturing-industries-1/assessment. The nutrient emission intensity here is measured in kilograms of nutrient equivalent released per million euros gross value added.

Nutrient emission intensities showed downward trends in the domestic sector and the manufacturing industry during the assessed periods. Similarly, nitrogen (nitrate) and phosphorus (orthophosphate) concentrations in rivers declined (Figure 2.3).

Figure 2.3 Nutrient trends in European rivers.

Source: http://www.eea.europa.eu/data-and-maps/indicators/freshwater-quality/freshwater-quality-assessment-published-may-2

1992-2012: Austria, Belgium, Bulgaria, Germany, Denmark, Estonia, Finland, France, Ireland, Latvia, Liechtenstein, Lithuania, Luxembourg, Norway, Poland,  Slovakia, Slovenia, Sweden, Switzerland, United Kingdom.

2000-2012: Austria, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Cyprus, Denmark, Estonia, Finland, France, Germany, Iceland, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Former Yugoslav Republic of Macedonia, Netherlands, Norway, Poland, Romania, Serbia, Slovakia, Slovenia, Sweden, Switzerland, United Kingdom.

In European rivers, the average concentration of orthophosphate fell by more than half over the period 1992–2012. This can be attributed to measures taken under the UWWTD, in particular the removal of nutrients during wastewater treatment. In addition, the introduction of phosphate-free detergents has contributed considerably to the reduction of phosphate concentrations in surface waters.

During the past few decades, phosphate concentrations have also gradually reduced in many European lakes. Many outlets from wastewater treatment plants have been diverted away from lakes to rivers and this has led to significant phosphate reduction.

The assessments of the first river basin management plans under the WFD showed that diffuse pollution from agriculture constitutes a significant pressure for more than 40 % of Europe’s rivers and coastal waters, as well as for one third of Europe’s lakes and transitional waters (EEA, 2012).

Intensive use of mineral fertilisers and manure leads to high nutrient surpluses often causing diffuse nutrient pollution of surface waters, and groundwater. While phosphate is adsorbed to particles in the soil, limiting its release into water, nitrogen compounds, especially nitrate, can dissolve in water and seep through the soil layers, leaching into groundwater.

Figure 2.4 shows the nitrate concentration in Europe’s groundwater from 1992 to 2012. The shorter time series shows a similar pattern, with hardly any overall trend, although this larger selection of groundwater bodies shows a slightly higher average concentration level.

Natural nitrate levels in groundwater are generally very low (typically less than 10 mg/l NO₃), although this depends on the oxygen concentration, since low oxygen availability favours nitrite over nitrate. Nitrate concentrations greater than natural levels are caused entirely by human activities, such as agriculture, industry, and domestic effluents.

Figure 2.4 Nitrate concentration in Europe’s groundwater.

Source: http://www.eea.europa.eu/data-and-maps/indicators/nutrients-in-freshwater/nutrients-in-freshwater-assessment-published-6

1992-2012:  Austria, Belgium, Bulgaria, Denmark, Estonia, Finland, Germany, Ireland, Liechtenstein, Lithuania, Netherlands, Norway, Portugal, Slovakia, Slovenia.

2000-2012: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Ireland, Italy, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Portugal, Serbia, Slovakia, Slovenia, Spain, Switzerland, United Kingdom.

2.3 Impacts of nutrient pollution and eutrophication

2.3.1     Eutrophication in bathing waters

Algae (i.e. phytoplankton) are most abundant in the summer. Their abundance is linked to the availability of nutrients in the water. Nutrient enrichment/eutrophication may therefore cause excessive growth of plankton algae (i.e. increase in phytoplankton biomass) which increases the concentration of chlorophyll a. This in turn may result in an increase in frequency and duration of phytoplankton blooms, which can pose hazards to human health in the case of toxic blue-green algae (EEA, 2012).

In marine and coastal waters, the chlorophyll a concentration is a proxy for the primary production and indicates the amount of phytoplankton. The higher the concentration, the higher the biomass of algae (Figure 2.5).

Figure 2.5 Chlorophyll a concentrations in European seas.

Source: http://www.eea.europa.eu/data-and-maps/figures/map-of-summer-chlorophyll-a-concentrations-observed-in-2.

Chlorophyll a concentrations are moderate or high in most European seas. According to reported data between 1985 and 2012 (http://www.eea.europa.eu/data-and-maps/data/waterbase-transitional-coastal-and-marine-waters-10), concentrations are decreasing in the Greater North Sea, Bay of Biscay and Adriatic Sea, but increasing in many parts of the Baltic Sea. The species that make up the pelagic food web and how it functions may change in ways that exacerbate the negative effects of excessive phytoplankton growth. Eutrophication in seas can also promote harmful algal blooms that may discolour the water, produce foam, kill off benthic fauna and make fish or shellfish poisonous to humans. Furthermore, excessive algal blooms in coastal waters (as well as in freshwater) could also affect tourism, with immense effects on the economy of the regions concerned.

Highly polluted lakes often have harmful blue-green algal blooms in summer. Optimal conditions for blue-green algae include intense radiation, high temperature, high levels of nutrients (phosphorus and nitrogen) and a lack of flow or turbulence. Blooms could endanger human health because of the algal toxins. They can cause rashes, skin and eye irritation, allergic reactions and other effects.

Blue-green algae produce microcystin, which is a toxin. WHO sets standards of 1 μg/l for microcystin in drinking water (WHO, 2003a). If the level exceeds this value, additional treatment is necessary, or drinking water should be taken from a different source (see also Annex A3).

2.3.2     Nitrogen in drinking water

The DWD sets standards for drinking water of (a) 50 mg/l of nitrate (NO₃) and (b) 0.5 mg/l of nitrite (NO₂). This owes to the fact that nitrate can subsequently turn to nitrite in the human body. Nitrite oxidises the iron in the haemoglobin of the red blood cellsthereby forming methaemoglobin, which lacks the oxygen-carrying ability of haemoglobin. This can create the condition known as methaemoglobinaemia (sometimes referred to as ‘blue baby syndrome’), in which blood cannot carry enough oxygen to the cells, so the veins and skin appear blue. The health concern is primarily if infants drink contaminated water (WRC, 2014).

In Europe, about 50 % of drinking water is taken from groundwater and about 30 % from surface water. Many waters in Europe are polluted with nutrients, and the levels of nitrate in groundwater have not decreased over decades. This might affect the supply of high-quality drinking water in the future, because the purification of highly contaminated raw water is becoming increasingly difficult and expensive. Increasing costs of water treatment could therefore also raise the price of drinking water for consumers.

2.4 Policies and measures to reduce nutrient pollution

The WFD is the key instrument for protecting water and setting quality objectives with respect to pressures such as pollution from diffuse sources (e.g. agriculture) or point sources. The UWWTD is one of the most important basic measures alongside the WFD and relates largely to point sources. Implementing the UWWTD has led to an increasing proportion of the EU’s population being connected to municipal treatment works via sewer networks. Furthermore, treatment levels have increased, moving from mechanical to biological treatment or to more stringent treatment (N and P removal). However, although there are national regulations that set standards for small waste water treatment plants (less than 2000 p.e.), neither the UWWTD nor any other European legislation directly addresses small-scale sanitation, which is inadequate in some locations. This poses a potential threat to water quality and public health particularly in rural areas (EEA, 2015).

Besides urban wastewater treatment, cost-effective measures to tackle diffuse agricultural pollution are also triggered through the WFD and its river basin management plans where water bodies are not in good status. The WFD covers the requirements for protected areas under the NiD, the BWD and the UWWTD, which include restrictions on water use, in particular for agriculture.

Box 2‑1 Good practice: two examples

Blue-green algae bloom in a reservoir

1) Pollution of a reservoir in Hesse, Germany, was found to be caused by significant nutrient discharges from six small-scale UWWTPs on the two tributaries to the lake. The discharges caused blue-green algal blooms and repeated fish kills between the year 2000 and 2009. Subsequently bathing was banned in the summer months. This led to a reduction of income from tourism and so the local authorities and other concerned parties developed a common strategy to combat eutrophication of the lake. The following objectives were set: (a) halving the concentration of phosphate in the discharges from all six waste water treatment plans, (b) merging two treatment plants and in addition diverting wastewater to another plant with higher treatment capacity, (c) reducing the percentage of water from external sources and (d) refurbishment of the sewer collection system. These measures have greatly improved the trophic condition in the lake. Since 2010, there have no further algal blooms.

2: In the Federal State of Baden-Württemberg, Germany the “Protected Areas and Compensatory Regulation” (Schutzgebiets- und Ausgleichsverordnung - SchALVO) was adopted which aims to protect the raw drinking water from agricultural nutrient and pesticide contamination. The regulation restricts land use practices in the water protection areas according to different zones: zone I, ‘nitrate sanitation areas’; zone II, ‘nitrate problem areas’; and zone III, ‘normal areas’. The ‘water cent’ is an additional tax on the consumer price for drinking water to compensate farmers for implementing agri-environmental measures and potential losses in income (Umweltministerium BW, 2001).

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]

Source: http://www.eea.europa.eu/data-and-maps/indicators/freshwater-quality/freshwater-quality-assessment-published-may-2

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.

2.6 Stormwater overflow from combined sewers

Combined sewers transport urban waste water as well as urban surface runoff in one pipe. Large amounts of rainfall in a short period can exceed the design capacity of the combined sewer leading to a combined sewer overflow (CSO). If the retention capacity of the sewers or the waste water treatment plant is insufficient, the excess water can cause flooding and untreated waste water is released.  

Organic substances, hydrocarbons, chemicals, heavy metals, litter and pathogens (e.g. bacteria, viruses, parasites) in the released waste water  can adversely affect water quality and aquatic life and impair bathing or the use of drinking water from nearby drinking water supply zones. CSOs lead to a multiple, diffuse and uncontrolled source of pathogens and pollutants in surface waters. It is one of the major threats for the quality of bathing waters (see chapter 3), and therefore for human health (Cools et al, 2016). Chemicals in CSOs may cause oxygen depletion, nutrient enrichment/eutrophication and toxic effects for aquatic organisms. These can result from pesticides, herbicides, fertilisers and other substances commonly applied to urbanised areas, farmlands and suburban gardens.

There are no EU-wide statistics on the actual number of CSOs at the agglomeration or at the national level. Ten EU Member States (Austria, Bulgaria, the Czech Republic, Finland, France, Germany, the Netherlands, Slovakia, Sweden and the United Kingdom) reported in 2010 under the WFD reporting that CSOs exerted a significant pressure on the ecological status of surface waters in their river basin districts.

As the implementation of the UWWTD is now quite advanced, it seems likely that efforts to improve ecological status under the WFD will bring more focus on the management of CSOs. By nature it will not be possible or economically feasible to design sewers to cope with all extreme events, but management measures can try to optimise the retention capacity of CSOs. Management solutions need to  control the frequencies, volumes and pollutant loads to the receiving waters.

In the future, the impacts of storm water overflows could also be influenced by climate change, as changes in precipitation can influence the frequency and volume of discharges from CSOs. In the future, more intense rainfall events are expected particularly in Northern and Central Europe and these might cause more frequent overflows and with larger volumes. In addition, urban developments and increased sealing of surfaces could exacerbate the problem and result in even higher run-off and greater risk of storm water overflows (Moreira et al, 2016).

2.7 Emerging issues

In addition to nutrient and organic pollution there are a number of emerging issues some of which are as yet poorly understood in their potential impacts to public health and the integrity of aquatic ecosystems.

Emerging pollutants are typically chemicals present at low concentrations but which may have harmful effects on aquatic organisms or those that feed on them. They are not necessarily new chemicals. Emerging pollutants can be defined as pollutants that are currently not included in routine monitoring programmes at the European level and which may be candidates for future regulation, depending on research on their (eco)toxicity, potential health effects and public perception and on monitoring data regarding their occurrence in the various environmental compartments. Examples from the list of emerging substances are surfactants, flame retardants, pharmaceuticals and personal care products, gasoline additives and their degradation products, biocides, polar pesticides and their degradation products and various proven or suspected endocrine disrupting compounds (EDCs) (http://www.norman-network.net/?q=node/19). Such pollutants are not currently covered by EU legislation but the WFD provides a mechanism by which concentrations in the aquatic environment can be controlled.

Plastic in the environment is another emerging issue, especially the impacts of micro-plastics in the environment. Micro-plastics are pieces of plastic than have been scoured and degraded and are generally defined as being less than 5 mm in size. They have been recognised as a widespread contaminant in the marine environment (GESAMP, 2015) but their relevance is being increasingly discussed also for freshwaters. Research into the impacts from micro-plastics is under way. So far, European water legislation does not control the sources of micro-plastics, their pathways from land through wastewater treatment plants or from diffuse sources. 

Antimicrobial resistance (AMR) is a phenomenon highly relevant for human health. It refers to the acquired resistance of a microorganism to an antimicrobial drug that was originally effective for treatment of infections caused by it. Resistant microorganisms are able to withstand attack by antimicrobial drugs (e.g. antibiotics), so that standard treatments become ineffective and infections persist, increasing the risk of spread to others (WHO 2015). This novel risk in the aquatic environment is at an early stage of research, but some recent work shows that increases in number of AMR bacteria could be associated with the proximity to wastewater treatment plants (Amos et al 2014, 2015).