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Introduction

Nutrient compounds frequently present in wastewater are valuable substances which act as fertilizers. They are becoming increasingly significant in water and wastewater management because the discharge of nutrients such as nitrogen and phosphorus into rivers and lakes can cause adverse influences on our environment and life. An excessive increase in the quantities of these nutrients in the aquatic surroundings disturbs the ecological balance, resulting in severe damage to environment (e.g. eutrophication).

It is probable that either nitrogen or phosphorus will be the limiting nutrient controlling eutrophication because of the relatively large quantities required for biomass growth compared to other nutrients, such as sulfur, potassium, calcium and magnesium.

Nitrogen is dissolved in water as ammonia, nitrite and nitrate and is present in organic molecules such as amino acids, which are formed by the hydrolysis of proteins and are transformed to ammonia during biodegradation. Ammonia and organic nitrogen compounds are most closely associated with plants and animals. An example of an organic nitrogen compound is urea (NH2CONH2), which is a major chemical component of urine. Urea is produced from ammonia in fauna and converted to ammonium by hydrolysis.

Several problems result from discharging wastewater with ammonia and nitrate into rivers and lakes:

• Ammonia is oxidized by bacteria to nitrite and nitrate, leading to a decrease in the dissolved oxygen concentration and to fish killing.

• Uncontrolled nitrification of ammonia causes a decrease in pH in the receiving stream.

• Ammonia and ammonium are in chemical equilibrium; with increasing temperature and pH more and more ammonia is produced which is toxic to fish.

• Nitrate stimulates the growth of algae, contributing to the eutrophication of open bodies of water.

• Nitrite and nitrate may reach groundwater resources which are used for producing drinking water. High concentrations of nitrate and nitrite in drinking water cause methemoglobinemia in babies and promote the formation of carcinogenic nitrosamines. As a result, the water must be denitrified in drinking water plants.

• In oxygen-free soil layers, denitrification can cause sludge build-up and anaerobic decomposition, resulting in the generation of methane.

In order to solve these problems, nitrogen must be removed from water. Biological nitrification and denitrification are an alternative.

Phosphorus is a key element in all known forms of life and a common earth element which can be induced into aquatic ecosystems by natural and human-caused erosion of soil materials and by human activity, e.g. the use of fertilizer in agriculture. It exists in different forms, such as dissolved inorganic orthophosphate, dissolved organic phosphorus found in algae, dissolved inorganic polyphosphate and non-dissolved particulate phosphorus (Fig. 10.1):

• Dissolved organic phosphorus is found as a lysis product of algae and bacteria in water and is used for industrial products like pesticides, complex binders and antiknock agents. They are difficult to biodegrade and pass through bank filtration and the filtration of water purification plants (Klinger 1999).

• Two types of dissolved inorganic phosphates are orthophosphate and polyphosphate. Orthophosphate takes the form of PO3-, HPO^- or H2PO-, depending on the pH value. PO^ plays a major role in organic molecules such as DNA and RNA, where it forms part of their structural backbone (see Fig. 3.8 in Chapter 3). Living cells also utilize phosphate to transport cellular energy via adenosine triphosphate (ATP; see Fig. 3.15). Existing orthophosphate facilitates algal growth. This is followed by algal death, lysis of algae and biodegradation by aerobic bacteria, which leads to oxygen depletion in lakes (eutrophication). Orthophosphate is stored in algae as polyphosphate. Polyphosphate is formed by polymerization of orthophosphate linked between hydroxyl groups and hydrogen atoms.

Total phosphorus Pt

Particular phosphorus PP (organic & Inorganic)

Dissolved phosphorus Pd

Dissolved polyphosphate Ppd (inorganic)

Hydrollzed dissolved phosphorus Pdh

Orthophosphate PO4-P (inorganic)

Dissolved phosphorus Pox (organic)

Fig. 10.1 Different forms of phosphorous in wastewater (Klein 1988; DIN 38405, D11).

Fig. 10.1 Different forms of phosphorous in wastewater (Klein 1988; DIN 38405, D11).

• Non-dissolved organic phosphorus particles are found in organisms and their cell refuse. A part of these phosphorus particles is separated by sedimentation and filtration; the colloid particles are eliminated only after flocculation and membrane processes.

To avoid problems with nitrogen and phosphorus, more and more limitations are being placed on the discharge permits of WWTP.

The minimum requirements for the discharge of municipal wastewater into inshore waters are laid down by the Wastewater Framework Regulation (Abwasserverordnung: Verordnung über Anforderungen an das Einleiten von Abwasser in Gewässer) of the Federal Republic of Germany from 17 June 2004 (AbwV 2004) which was recently renewed, going into effect from 1 January 2005.

The NH4-N concentration of a 2-h mixed sample of domestic and municipal wastewater must be less than 10 mg L-1 NH4-N for BOD5 loads greater than 300 kg d-1 (Appendix 1 in AbwV 2004). That means that all treatment plants with flow rates greater than 4000 m3 d-1 have to be expanded to include a nitrification stage. An expansion is made necessary by the limit on the total inorganic nitrogen content (ammonia, nitrite and nitrate) and total phosphorus content. For BOD5 loads from 600 kg d-1 to 6000 kg d-1, the total N and P concentrations of a 2-h mixed sample must be less than 18 mg L-1 N and 2 mg L-1 P respectively. For BOD5 loads greater than 6000 kg d-1, both limit values are even lower, i.e. 13 mg L-1 N and 1 mg L-1 P (see Table 2.9).

The German Wastewater Framework Regulation also sets limits for industrial effluents for their direct discharge into inshore waters. The limit for total N ranges from 18 mg L-1 N for the food industry to 70 mg L-1 N for landfill leachate water. The limits for phosphorus and ammonium nitrogen are mostly fixed at 2 mg L-1 Pt and 10 mg L-1 NH4-N, respectively (Table 10.1).

The different limits for various branches of industry depend on the raw materials used. For example, most food producers have the same limits of total N and P concentration as those for domestic and municipal wastewater at the 6000 kg d-1 BOD5 level.

The European Union passed Directive 91/676/EEC (EU 1991a) concerning the protection of waters against pollution caused by nitrates from agricultural sources to reduce or prevent water pollution. The member states are obliged to take measures against the discharge of nitrate into surface waters and groundwater. Moreover, a framework for European Community action in the field of water policy was established in the form of Directive 2000/60/EC from 23 October 2000 which aims at maintaining and improving the aquatic environment in the EC. It was completed and amended by Decision No. 2455/2001/EC from 20 November 2001 to establish a list of priority substances (Annex X) in the field of water policy.

In addition, Directive 91/271/EEC (EU 1991b) requires the collection and treatment of wastewater, with P removal in sensitive areas and effectively in almost all large urban areas. Application of this Directive is essential to protect the quality of surface waters (see Chapter 2 for regulations concerning wastewater and Chapter 12 for hygienic standards for bathing water).

Table 10.1 German legal requirements for direct discharge of specific industrial effluents to inshore waters regarding nitrogen, phosphorus, COD and BOD5 (AbwV 2004).

Industry/products

SNH4-N SNt

(mg L-1 N) (mg L-1 N)

Spt (mg L-1 P)

(mg L-1 COD)

(mg L-1 BOD5)

Food production^1

10

18

2

110

25

Sugar production

10

30

2

200

25

Edible oil refinery

-

30

4.5

200

38

Leather production

10

-

2

250

25

Biological treatment of waste

-

70

3

200

20

Meat meal industry

-

50

-

150

25

Cellulose production

-

10

2

25

30

Gelatine production

10

30

2

110

25

Paper production

-

10

2

50

25

Textile production

10

20

2

160

25

Petroleum processing

-

40

1.5

82

25

Laundry

-

20

2

100

25

Animal and plant production

-

-

2

110

25

a) This includes milk, brewery, potatoes, meat, fish, drinks, alcohol and alcoholic drinks, fruits and vegetables.

a) This includes milk, brewery, potatoes, meat, fish, drinks, alcohol and alcoholic drinks, fruits and vegetables.

In domestic wastewater, one major problem is that the ratios of N:C and P:C of many organic compounds in wastewater are much higher than those needed by heterotrophic bacteria for catabolism and anabolism. Therefore, inorganic and organic N and P compounds are left in the treated wastewater. The processes for nitrogen and phosphorus removal are generally applied for domestic wastewater treatment. In industrial effluents, the contents of N and/or P are usually too low, so that N and/or P must be supplemented via additives. If the wastewater has a high N concentration, it is removed by stripping with steam or air at higher pH which must be cleaned afterwards, e.g. by absorption and reaction in sulfonic acid. The process of nitrification and denitrification has been used here only seldom.

The typical mean NH4-N and total N concentrations in raw municipal wastewater range is 44.5-75.9 mg L-1 NH4-N and 74.5-103.5 mg L-1 N in Berlin wastewater (WWTP Ruhleben). The total P concentrations range is 11.7-18.9 mg L-1 P (BWB 2004; see also Table 2.3). But sometimes industrial effluents are heavily loaded with ammonia; and its concentration varies depending on the production processes responsible (see Table 2.4).

Chemical systems have frequently been used to remove phosphorus in wastewater treatment. Biological processes to remove nitrogen and phosphorus from wastewater have become more or less standard technology in wastewater treatment. The utilization of biological nutrient removal processes for the treatment of wastewater has environmental, economical and operational benefits. We will return to this topic later.

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