Application of Nitrogen Removal by Precipitation

Nitrogen removal by precipitation of magnesium-ammonium-phospate has not yet found a full scale application, but it cannot be excluded that the process will be used in the nearest future for industrial waste water of the right composition to allow an economical removal of phosphorus and nitrogen at the same time.

Schulze-Rettmer (1991) has examined the process in details and finds that it is an attractive method to use for nitrogen removal, from a technical as well as from an economic's point of view. He calculates that the removal of 1 kg ammonium- N by precipitation as magnesium-ammonium-phosphate, using magnesium oxide and phosphoric acid as chemicals, will cost about 5 U.S. Dollars. The costs are reduced if the waste water contains significant quantities of phosphate and magnesium. The cost of chemicals is estimated to be 70% of the total costs. This implies that a reduction of the ammonia concentration in municipal waste water from 40 mg ammonium-N /1 to 5 mg ammonium- N /1 will cost about 25 U.S. cents / m 3, which is comparable to nitrification and denitrification. The value of the magnesium ammonium phosphate produced by this process can be estimated as 12 U.S. cents / kg, considering the purity of the product, compared with 25 U.S. cents / kg for the usually applied technical quality. The conclusion from this review of the process by Schulze-Rettmer is that the precipitation of ammonium-N as magnesium-ammonium-phosphate is economically feasible and should be considered as a serious alternative to other nitrogen removal processes.

Precipitations of proteins have, however, been widely used. A discharge fee for waste water related to the concentrations of pollutants has been introduced in many countries, i.e. the fee is found on basis of BOD5, COD, phosphorus and/or nitrogen concentrations in the effluent. This has provoked many industries and in particular food industries to introduce a waste water treatment, which is able to reduce the concentrations of BOD5, COD, phosphorus and / or nitrogen to the level of municipal waste water. The Industries are thereby able to reduce their discharge costs considerable. It can be shown that the costs of the treatment including depreciation and interest of the treatment plant often are much lower that the discharge costs, which makes it profitable for the industries to introduce treatment of the effluent.

Recovery of proteins gained by precipitation of industrial waste water is, unfortunately, only accomplished in few industries. Some industries deliver free of charge the protein-rich sludge to meat-bone-meal factories, where the sludge is treated as other waste, which is the raw material for the production. As it is expected that dumping of any solid waste product will be more and more limited in the future, the use of the sludge from treatment of food processing waste water for production of animal feed will probably become more and more attractive. The general development seems clear for industrial waste water: from no treatment, to treatment due to high discharge fees and finally to recirculation and recovery of waste products.

Figure 11.21 is a flow diagram of the combination of chemical precipitation and ion exchange used in the treatment of waste water from the food industry

(Jorgensen, 1971, 1973, 1976 and 1978). This process allows recovery of fat, grease and proteins. Table 11.2 gives the analytical data obtained when this process was used on waste water from herring filleting after centrifugation of the raw waste water to recover fish oil. Table 11.3 gives the analyses of this process for waste water from an abattoir. For comparison Table 11.13 includes the results obtained by using a biological plastic filter.

It can be concluded from these results that the application of chemical precipitation to waste water from the food processing industry is advantageous to use to reduce the pollution to or almost to the level of municipal waste water. The process is able to reduce the nitrogen concentration of these types of waste water considerably and can therefore be considered as an attractive method for the removal of nitrogen, although the method is most often selected because of its over-all effect of BOD-5, COD, P and N-reduction. The method is simultaneously a practical method for recovery of proteins and it is expected that this feature of the process will become increasingly important in the coming years.

Cl2?

Precipitant

Ion Exchanger

Elution liquid

Screening

Flocculation

Sludge'

Recovery of proteins (+grease)

Figure 11.21 Recovery of proteins (+ grease).

Table 11.2.

Analytical data of waste water from herring filleting

Table 11.2.

Analytical data of waste water from herring filleting

Raw

After cen-

After chem.

After Cel

waste

trifuga-

precipi

lulose ion-

water

tion

tation

exchanger

1. step

2. step

3. step

BOD5 (mg/l)

11000

5800

2000

1100

N (mg/l)

180

162

60

23

Susp, matter (mg/l) 400

170

40

2

KMn04 (mg/l)

8000

4000

1200

Analysis of waste water from an abattoir (mg/l)

Table 11.3

After chem.

After chem.

After biolo

precipitation

precipitatior

Raw

gical plastic

(glucose sul

and ion ex

water

filter

fate is used)

change

bod5

1500

400

600

50

KMn04

950

350

460

60

Total N

140

42

85

15

hn3-n

20

15

18

2

no3¬Ľn

4

5

4

1

P

45

38

39

1.5

APPENDIX of PART B: DESIGN EXAMPLES

Determination of kinetic coefficients

APPENDIX B 1. DETERMINATION OF KINETIC COEFFICIENTS k, Ks, Mmax, Yobs AND Kd FROM LABORATORY DATA.

Data are derived from a high-strenght bench-scale mixed activated sludge reactor without recycle, show the following substrate concentrations.

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