CS277618B6 - A method for removing hexamethylenetetraamine from wastewater - Google Patents

Abstract

The solution concerns the biological removal of hexamethylenetetramine from wastewater in an acidic environment. The essence of the solution is that the acidic environment is maintained by constantly removing settled activated sludge in an amount of no more than 1/10 of the total activation volume.

Description

The invention relates to the removal of hexamethylenetetramine from wastewater by biodegradation in an acidic medium.

Prior art

Hexamethylenetetramine (HMT) is a colorless crystalline compound used in the manufacture of plastics, disinfectants and explosives. It is very soluble in water and, as a component of wastewater, is very difficult to remove by biological treatment methods. The cyclic structure of the HMT molecule is extremely stable and cannot be degraded by enzymatic processes. The only option is to hydrolyze the HMT molecule in an acidic environment. The reaction proceeds according to equation / 1 /.

/ CH _2/6 N ₄ + 4H ⁺ + 6 H ₂ 0 = 4 NH + HCHO 6/1 /

The resulting formaldehyde is already biodegradable. The rate of hydrolysis is slow and thus decisive for the efficiency: removal of HMT. Degradation rates of HMT - 1 - Ί in the range of mg.g units are reported in the literature. h at a pH of the activating mixture of from 5 to 6.5. For this reason, it is necessary to accelerate the hydrolysis at high concentrations of HMT in wastewater (more than ¹ g.1 -1). However, for lower concentrations of HMT (hundreds of mg.I ^-1 ) it is not necessary to catalyze the hydrolysis in waters with undesired NO ₂ . In this case, it is sufficient to maintain: the pH of the activation in the acidic range. The simplest way is to add acids or mix with other acidic wastewater. The addition of various chemical agents to the water causes salinity of the water and thus further pollution of the environment. The literature describes a process for the pre-treatment of waste water containing bound formaldehyde by adding nitrite to the waste water by direct metering or in the form of a solution in an amount of at least 30 mg of nitrite per 1000 mg of bound formaldehyde at pH 2.5 to 4. Under these conditions, the rate of hydrolysis increases several times. At the same time, the rate of HMT re-synthesis, which should be considered, slows down: after increasing the pH.

The essence of the invention

This problem can be solved: by a method of removing hexamethylenetetramine from wastewater by biodegradation in an acidic medium according to the invention, the essence of which is that the acidic medium is maintained by constantly removing settled activated sludge in an amount of at most 1/10 of the total activation volume. The acidic environment in the wastewater is created by nitrification of the NH ₄ present with nitrifying bacteria, the amount and activity of which depend on the age of the sludge, which must be: at least 10 days.

A necessary condition for the functioning of the system is the creation and maintenance of acidic pH by nitrifying activity. Wastewater containing HMT faces usually contains higher concentrations of NH ₄ and at the same time NH _{4 is} released from HMT molecules during the hydrolysis itself. Nitrification proceeds according to the reaction / 2,3 /

NH ^ ·

1.5 0 ₂

2H ⁺ + H ₂ 0/2 / nhJ ·> NO ₃ +

2H ⁺ + H ₂ 0/3 /

From the reaction (2, 3) and from the stoichiometry of the reaction (1) it follows that from 1 mmol of HMT, i.e. 140 mg, 72 mg of NH _{4 are} formed and after nitrification 4 mmol of H ⁺ .

The main problem with this method of maintaining an acidic environment and at the same time removing HMT is the activity of microorganisms. All nitrifying bacteria are significantly inhibited by the acidic environment. Nevertheless, cases are known where nitrification takes place even at a pH lower than 5.4. The specific rates of nitrification are very slow and it is therefore necessary to increase the concentration of nitrifying bacteria as much as possible. An increase in the number of nitrifying bacteria was achieved by maintaining the older age of the sludge.

The advantages of the solution according to the invention lie primarily in allowing the hydrolysis of HMT and the subsequent biodegradation to take place, while using the components of wastewater pollution and without the need to add other chemical compounds. This also prevents salting of the water. The problem of removing the present and hydrolysis of the released NH _{4 is also} partially solved.

Examples of embodiments of the invention

Example 1

The composition of the substrate used was as follows:

HMT = 180 mg.I ^-1 , cyclohexanol = 400 mg.l ^-1 , pH = 7.0,

KNK ₄ / acid neutralizing capacity at pH 4.5 / - 5 mmol.l ^-1 , NH ₄ = 40 mg.l ^-1 .

The mixing activation model had the following parameters:

volume 4.4 l, waste water residence time 100 h, recycle ratio 1. 1/20 volume, i.e. 220 ml, was taken daily from the activation tank, thus maintaining the age of the sludge in the system for 20 days. After 50 days of activation operation, the following values were measured at the activation effluent: pH 6.2 to 6.3, HMT concentration 118 mg.l ^-1 .

The activation system thus characterized showed a 38% HMT removal efficiency.

Example 2

The conditions were the same as in Example 1, except that the age of the sludge was increased to 200 days. The pH in the effluent was 4.8 ± 0.3 and the concentration dropped to 50 mg. ” ¹ , ie the HMT removal efficiency was higher than 70%.

Industrial applicability

The process according to the invention can be used for all types of waste water containing hexamethylenetetramine which do not contain nitrification inhibitors at the same time.

Claims (1)

Process for the removal of hexamethylenetetramine from waste water by biodegradation in an acidic medium, characterized in that the acidic medium is maintained by the continuous removal of settled activated sludge in an amount of at most 1/10 of the total activation volume.