Industriernes spildevandsudledning i byernes økologiske kredsløb

Industrial Sewage Discharges in Urban Ecology

Summary and Conclusions

Aims for residual sludge reuse

The aim of the Danish government is that 50% of the sludge produced at Danish wastewater treatment plants should in 2004 be reused as manure on agricultural land. In 2001 52.7% of the sludge from the treatment plants was reused for this purpose.

Limits

In to be used for agricultural purposes, sewage sludge must satisfy several conditions in terms of heavy metal and persistent organic pollutant content. These conditions have continually been tightened, which means that sludge from a number of treatment plants cannot be reused as manure on agricultural land.

Sources of pollution

Sources for sewer system pollutants containing heavy metals and organic pollutants are households, rainfall runoff and industry. This report focuses on the discharge of pollutants from industry and on the initiatives that have been implemented to reduce these discharges.

Development in sludge quality

Since the late eighties the concentration of heavy metals in sludge from wastewater treatment plants in Denmark has generally been falling, probably as a result of the increased focus on pollution reduction in this period. Also the contents of the organic pollutants Polyaromatic Hydrocarbon (PAH), Di(-ethylhexyl)phtalate (DEHP), Nonylphenolethoxylate (NPE) and Linear Alkylbenzene Sulphonates (LAS) in sludge were reduced from 1997 to 2001. This reduction is largest for NPE, while the reduction of the LAS concentration is very small.

Discharge from industry

When comparing sludge from different wastewater treatment plants observations reveal a slightly decreasing content of mercury and copper as the share of industrial sewage treated in the plants increases . The organic pollutant PAH exhibits a weak tendency towards increased concentrations with increased industrial sewage fractions. No relationship is observed between sludge quality and the fraction of industrial wastewater for the other pollutants discussed in this report.

No general relationship between industry and sludge quality
The treatment plants dealt with in this report are regarded as representative of the Danish treatment plants. Therefore it is concluded that for the majority of the pollutants there is generally speaking no relationship between individual sectors of industry and pollutant residual in the sludge. This conclusion is based on information from the entire country. However, locally there may be a relationship between specific industrial sectors and contamination with heavy metals and organic pollutants.

This relationship is illustrated in case studies, where discharges from specific industrial sites constribute a major share of the total load on the treatment plant.

Influence of rainfall runoff

The relationship between the proportion of separate sewerage in the catchments and the resulting sludge quality has also been studied.

A higher lead concentration was found in sludge originating from treatment plants treating wastewater from catchments served by a combined sewer. This tendency is probably due to runoff from lead roofs, which are known to contribute significantly to lead in wastewater. Nickel and PAH exhibit the same tendency, but to a lesser extent.

Heavy metals can often be traced

Although generally speaking there is no general relationship between industrial activity and the heavy metal content, many wastewater treatment plants have been able to trace high concentrations of heavy metals to specific industrial sites. This is partly because the heavy metals are not broken down in the sewage system or at the treatment plant. The fraction of heavy metals that can be found in the sludge depends on whether the metals accumulate in the sewer system and on the efficiency of the treatment process at the wastewater treatment plant.

Difficult to trace organic pollutants

The difficulties in relating organic pollutant concentrations in the sludge to concentrations found in the wastewater are presumably due to differences in the treatment plant configuration. Whether the sludge is stabilised under anaerobic or aerobic conditions has a very large influence on the sludge quality. Aerobically stabilised sludge has a much lower concentration of especially LAS and NPE than anaerobically stabilised sludge. Similarly, the use of a primary tank has been observed to have a major effect on sludge quality. When the primary tank was removed at BOV Centralrenseanlæg, the concentrations of LAS, NPE, PAH and DEHP in the sludge were reduced by 95%, 95%, 85% and 80% respectively.

Case studies

During the project a number of counties, municipalities and associations were contacted in order to get an overview of the complexity of problems concerning sludge quality. In the light of these contacts, a number of interesting cases in Denmark were identified. These case-municipalities experience or have experienced problems related to heavy metals or organic pollutants in the residual sludge. The cases, the procedures and the initiatives that have been performed in order to reduce the pollution are described in this report.

Approaches to problem solving

In the cases studied, the following methods were used to solve the problems:

Dialogue with the polluting industry
Limiting values and terms specified in the connection permit
Special fee for the discharge of heavy metals to the sewage system
Campaigns targeted towards specific industrial sectors and the ordinary consumer
Operational key figures are set up for industries, to elucidate consumption and discharge
Phasing out of pollutants
Substituting pollutants
Wastewater treatment at local plants
Collection of wastewater which is subsequently transported to a waste disposal site (Kommunekemi)
Reduced use of products containing organic pollutants
Correct dosage of for instance cleaning agents

Large differences in sludge quality

There are large differences between the heavy metal and organic pollutant content in sludge originating from different treatment plants. The low concentration values at some plants suggest that other plants performing less successfully should be able to reduce their concentrations to similar levels. The high levels may be due to certain industrial sectors, specific industrial discharges and usage in private households. The wastewater treatment and stabilisation processes are of vital importance to the content of the organic pollutants LAS, NPE, PAH and DEHP in the sludge. Since it is not always obvious which sources are the most important, all sources must be monitored, for instance by control of connection permits, source tracing, inspection visits to industrial sites, and campaigns targeted towards specific industrial sectors and towards consumers.

Recommendations

In spite of the fact that, generally, industry does not contribute more to pollution with heavy metals or organic pollutants in residual sludge than for instance private households, it is recommended that, if problems with the quality of the sludge are experienced or if a specific industry is suspected of disproportional pollution, efforts are made to trace the sources of pollution. When an attempt to trace sources is initiated, it may be appropriate to start by mapping all relevant industries in the area. If sources are identified, the described methods for solving the problem can be used.

Source control

In spite of possible municipal decisions not to use sludge for agricultural purposes, efforts should be made to limit pollution at the source. Heavy metals and organic pollutants do not only contaminate the sludge, but also inhibit the treating processes at the treatment plant and contaminate both the recipients during sewer overflow discharges and the residual products from sludge incineration.