CS257618B1 - A method for removing ammonia and sulfides from water - Google Patents

Abstract

The method of removing ammonia and sulfides from water by stripping with air, steam or inert gas consists in removing both components from the water simultaneously at a pH of 9.0 to 11.0, preferably at a pH of 9.25. The stripped components are absorbed in a pair of absorbers

Description

The invention relates to a process for removing ammonia and sulphides from water.

Ammonia, together with sulfides, contributes to the pollution of a wide range of industrial waste water. A typical example is coke and gas wastewater, in which both components are present in high concentrations (of the order of grams to tens of grams per liter).

In order to allow such waters to be discharged into the recipient or to be biologically treated with other waters, it is necessary to reduce the ammonia and sulphide content to the values prescribed by the water authorities, or to reduce their concentration so as not to negatively affect the biological treatment.

The most commonly used methods of removing ammonia and sulphides from water include physicochemical methods, in particular steam or air stripping, oxidative methods such as breakpoint chlorination or ozonization, ion exchange, and others.

The principle of removing ammonia and sulphides from water by stripping with air can be expressed by two equations:

pH > -7

NH 4 + OH 4 NH 4 + H ₂ 0 pH <7 pH5> 7

H _? S ^> HS + H ⁺ pH * 7

The first equation expresses the effect of the alkaline reaction on the conversion of ammonium ion (dissolved ammonia) to free ammonia (ammonia gas). It has been found that at 20 ° C and pH 11, the free form constitutes about 95% of the total ammonia present in the water. Free ammonia can be easily stripped from the water with air or steam. If the water also contains phosphates, the ammonium phosphate co-precipitates at approximately the same optimum pH as the stripping.

The second equation shows that effective removal of sulfides from water affects low pH, below 7.

From the above finding, it can be concluded that the combined removal of both ammonium and sulfides from water is not possible. Typically, therefore, two-stage stripping is used at two different pH values or the process is performed in high towers / Klett R. J .: Hydrocarbon Processing Vol. 51, No 10, pp. 97, Darton R.C. et al., Chemical Engineers (England, No 339, 923).

In fact, both methods are a complicated solution and involve significant investment and operating costs.

The above-mentioned disadvantages are overcome by the air stripping method of removing ammonia and sulphides from the water according to the invention, characterized in that the two components are simultaneously removed from the water at pH 9.0 to 11.0, preferably at pH 9.25. The stripped components are absorbed in a pair of absorbers, the first for trapping ammonia in a solution of an acid such as sulfuric or nitric acid and the second for trapping hydrogen sulfide, for example in a sodium hydroxide solution.

The solution according to the invention makes it possible to remove both ammonia and sulfides from the water by a single stripping operation using a single packed column with a single pH adjustment at the beginning of the process. Initially, sulphides in the form of sulphate are desorbed more rapidly from water at pH = 9.25, and at the same time the pH rises to 9.8 to 9.9. After about 1/2 to ‘1 hour of stripping, the elimination of sulphides from water is complete. The pH increase favorably affects the stripping rate of ammonia. As the water content decreases, the pH drops to 6.2 to 6.4. This results in savings in both investment and operating costs over the methods used hitherto, which either require a double pH adjustment and two stripping columns or a high column with intermittent operation and deliver a pH adjustment.

The process of the invention is further described by way of example.

Example 1

Waste water with high ammonia content (9400 mg.l and sulfides) and 8900 mg.l was stripped with water vapor for 3.5 hours in a packed column laboratory model. The stripped ammonia was collected in a scrubber filled with boric acid solution, stripped with H 2 S in a scrubber with sodium hydroxide solution. The results of four experiments performed with waste water without pH adjustment and after treatment with sodium hydroxide solution are shown in Tab. AND.

Table I

Try C. pH mg. 1 ™ 3 ^C t mg: 1 9.1 9400 780 2 9.25 9400 20 May 3 10.00 9400 10 4 10.00 9400 5

efficiency % c ° 1 ¹ mg. 1 ^{Ct 1} mg. 1 efficiency % 91.7 8900 0 100 ALIGN! 99.8 8900 0 100 ALIGN! 99.9 8900 5200 41.6 99.9 8900 7100 20.2

c _q = initial concentration in water c = instantaneous concentration in water after 3,5 h.

Example 2

A waste water stripping pH of which was adjusted to 9.25 with sodium hydroxide solution was carried out once. Samples were taken from the distillation flask at regular intervals to determine the residual ammonia and sulphide content. The results are shown in Tab. II.

Table II

Time striping h nh ₃ with ² " IN mg. 1 efficiency % ^Ct -1 mg. 1 efficiency % 0 9400 0 8900 0 0.5 9250 1.6 8100 9 1.0 5600 40.4. 60 99.3 1.5 3420 63.6 0 100 ALIGN! 2,0 2140 77.2 0 100 ALIGN! 2.5 260 97.2 0 100 ALIGN! 3.0 20 May 99.8 0 100 ALIGN! 3.5 16 99.8 0 100 ALIGN!

c _t = instantaneous concentration in water

Claims (1)

OBJECT OF THE INVENTION Process for removing ammonia and sulphides from water by stripping with air, water vapor or inert gas, characterized in that both components are simultaneously removed from the water, at a pH of 9.0 to 11.0, preferably at a pH of 9.25, the stripped components are absorbed in a pair of absorbers, the first for trapping ammonia in a solution of an acid such as sulfuric or nitric acid and the second for trapping hydrogen sulfide, for example in a sodium hydroxide solution. Severography, n. P., MOST