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Degradation of Estrogens in Sewage Treatment Processes

2 Sewage treatment in Denmark

The treatment of municipal sewage in Denmark has during the last 15 years undergone significant improvements following the launching of a comprehensive national Danish Action Plan for the Water Environment (VMP I) in 1987. A follow-up action plan, VMP II, was initiated in 1998 and a further follow-up on that, VMP III, is presently being prepared and will be commenced in 2004.

The first VMP was initiated with the specific aim of significantly reducing the loads of nitrogen, phosphorus and organic matter (NPO) on the Danish water environment that in many places (coastal waters) up through the 1980'ies suffered from eutrofication and episodes of severe oxygen deficit with increasingly shorter intervals. One of the most important means to achieve this goal was to invest heavily in order to build new or modernise existing municipal sewage treatment plants (STPs) that could comply with new, stricter emission limits for NPO (8 mg/L, 1.5 mg/L and 15 mg/L, respectively).

Later, when the VMP II was prepared, increasing concern about heavy metals and other chemical pollutants in the environment lead to the launching of a national water environment monitoring programme, NOVA 2003, which, in addition to NPO, also included a large number of specific pollutants. These were in particular to be monitored at point sources. From 2004, a revised and extended version of the programme, NOVANA, will be in force.

Still, however, the designs and technologies of Danish STPs are aimed to minimise the discharge of NPO-substances rather than handling specific chemical compounds. This chapter is intended to provide a brief overview of the situation regarding STPs in Denmark.

2.1 Overview of STPs in Denmark

The national statistics on STPs only concern treatment plants being larger than 30 PE (1 PE is defined as 60 grams of BOD₅/day), which covers the vast majority of public sewage in Denmark as about 89% of the population is connected to such plants.

The number of STPs was 1,267 in 2002 of which more than 80% were operated by municipalities - including all the large plants - while the rest were run by private companies or organisations. The number of STPs at the beginning of the VMP I (1989) was 1,980 i.e. more than 700 plants have disappeared in about 13 years (DEPA 2003d).

The volume of sewage passing through the Danish STPs was about 810 million m³ in 2002 compared to 720 million m³ in 2001 (DEPA 2003d and 2002a). The figures include infiltration and surface runoff, the latter constituting about 100 million m³ in an average year (2001 is closer to the average than 2002, which was a year with much precipitation). Of this total volume, some 50-60 % was discharged directly into marine or brackish receiving waters while the rest was discharged to streams and lakes.

The size distribution of the Danish treatment plants was in 2002 as shown in Table 2-1 (DEPA 2003d).

Table 2-1
Size distribution of Danish STPs in 2002 (DEPA 2003d)

The discharge of NPO-substances in 2001 was dramatically lower than in the mid 1980'ies, i.e. prior to the launching of the VMP I. Thus, the organic matter discharges (as BOD₅) has been reduced by 95%, phosphorus by 92% and nitrogen by 79%. Table 2-2 below gives an overview of the NPO discharge in 1989 and 2001, respectively, and of the present (2001) average treatment efficiency of the Danish STPs (NERI 2002).

Table 2-2
nitrogen, phosphorus and organic matter in sewage from Danish STPs in 1989 and 2001 (NERI 2002).

2.2 Types of treatment technologies

The Danish STPs apply a considerable range of treatment technologies, which in terms of complexity are more or less directly proportional to their size. A simplified categorisation of 1,250 out of the total 1,267 Danish STPs larger than 30 PE is shown in Table 2-3 (based on data in DEPA 2003d. Specifications for 17 plants are missing).

Table 2-3
Overview of main treatment technologies at Danish STPs and their relative share of the volume of treated sewage.

M: Mechanical treatment
B: Biological treatment
N: Nitrification
D: Denitrification
K: Chemical precipitation

As could be anticipated, the highest efficiency (in terms of NPO removal) is found at the most advanced plants (Category A), which, at the national level, are also the most important in terms of volume and load (in PE) treated.

The plants in categories A-C all remove organic matter efficiently, i.e. more than 90%, while category D plants only remove some 55-70%. The figures for nitrogen removal are slightly lower, but the same trend between the different STP categories is observed. With regard to phosphorus only the most advanced plants (category A-B) remove this element with efficiency higher than 80% while the removal in category C plants is only some 50-60% and in category plants even lower; about 35-45%.

A typical, modern Danish STP is based on the activated sludge process. Most commonly, the plants operate according to the principle of recirculation or alternation. The following main stages will be found at most Danish STPs:

The first step in wastewater treatment is a mechanical step. The screen removes larger objects from the wastewater, whereas the grit and grease trap removes grit, grease and supernatant material. The discarded material is usually incinerated and the residues placed on landfills. Most large STPs are, in addition to the screen and trap, also equipped with a primary settling tank.

The biological treatment consists of a denitrification step (no oxygen present) and nitrification step (oxygen present). Nitrate is converted to free nitrogen in the denitrification step while in the nitrification step, ammonia is concerted to nitrate (via nitrite), and organic material is converted to carbon dioxide. To remove the nitrate formed in the nitrification step, the sludge is re-circulated back into the denitrification step.

As mentioned above some STPs are operated according to the principle of alternation. With alternation, the wastewater is treated in the same tank with shift between denitrification and nitrification, whereas operation according recirculation is moved from one tank (denitrification) to another tank (nitrification).

An anaerobic step for biological phosphorus removal is installed on many STP lately, but chemical phosphorus precipitation is still the most common method for phosphorus removal on Danish STPs.

The final step in wastewater treatment is separation (secondary settling tank) of activated sludge and treated wastewater. The treated wastewater is discharged to the receiving water.

The removed sludge is split into a fraction for sludge treatment and another fraction is transferred back to the biological step for maintaining of biological activity. Sludge treatment typically consists of sludge thickening and sludge dewatering. Approximately 45% (sludge production) of the Danish STPs are installed with anaerobic digester for further sludge treatment (DEPA 2003c). The excess sludge is mainly discharged for agriculture use (approximately 53% in 2001). Due to limits for heavy metals and other chemical pollutants the sludge amount for agriculture use are decreased during the last 8 years, whereas sludge discharged to incineration and other use are increased within the same period (DEPA 2003c).

Figure 2-1 shows a typical STP design based on recirculation.

Figure 2-1
STP based on recirculation principle.

2.3 The STP selected for this study

Activated sludge for the major part of the experiments performed in this study was supplied from the STP at Egå, a suburb situated a few km north of Denmark's second largest city, Århus. Egå STP treats sewage from the northern, suburban parts of Århus.

Egå STP is a so-called MBNDK type of sewage treatment plant, i.e. it treats sewage mechanically, biologically (under anaerobic, aerobic and denitrifying conditions) and chemically, and thus removes organic matter, nitrogen and phosphorus to a high degree. With a capacity corresponding to about 90,000 PE it is one of the larger Danish STPs, and it receives a higher than average load from industries and other enterprises (about 55% compared to an average of about 45% for other large STPs in Denmark). In the catchment is also a major regional hospital (Skejby Sygehus).

The total sludge retention time, SRT, at Egå STP is approximately 35 days (higher than for average Danish SRTs, namely 25-30 days) and the hydraulic retention time, HRT, for the STP is approximately 0.5-2 days (depending on volume of rainfall). The HRT in the anaerobic tank, denitrifying tank and nitrifying tank is on average 1.5, 6.5 and 6.5 hours, respectively.

The combined treatment of sewage at the plant reduces COD with 97%, total-N with 94% and total-P with 96%. The quality of the effluent (mean volume of 19,000 m³/day) appears to be somewhat better than the average for this type of STP (COD: 24 mg/L, BOD₅: 1.6 mg/L, total-N: 3.7 mg/L, total-P: 0.4 mg/L) and the performance parameters suggest that Egå is a well-run STP.

Egå STP was chosen for the study for a number of reasons:

1) It was fairly representative of larger Danish municipal STPs

2) It was already well described with regard to operational parameters and quality of inflow and outflow etc.

3) The plant was well known to the project team, thus making practical arrangements in relation to sampling easy and reliable.

4) Other related investigations had taken place and continue to take place in the catchments to and receiving water of Egå STP.

Regarding the last item, the choice of Egå STP as a source of sludge for the study, was based on the wish to provide synergy to the total work on estrogens and endocrine disruptors in Denmark. Thus, both the municipality and county of Århus had been proactive and previously engaged in work on the issue of hormone disrupting compounds. Additionally, the municipality was at that time initiating further environmental studies on their STPs and receiving waters, and had volunteered to fund the additional costs associated with sampling and transport etc for the sorption and degradation study.

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Version 1.0 November 2004, © Danish Environmental Protection Agency