The principles of ‘green chemistry’ using the example of design and operation of an underground water treatment station at a Moscow food processing enterprise for purification out of compounds of iron and the utilization of byproducts

This paper deals with the technology of ozonation together with sorption on activated carbons (Frog 2003) and its compliance with 12 principles of green chemistry (Lunin et al. 2005). The report also provides an illustrative example of a drinking water treatment station with a capacity of 160 m³/h which purifies underground water from ions of iron by using ozone sorption technology and 12 principles of green chemistry. R&D, design, construction, and maintenance of the station are also described.

The improvements of some chemical processes, which positively influence both the health of individuals and the environment and correspond to the main 12 criteria, are stated below in Table 1.

Ozone has a characteristic smell (Lunin et al. 1998), the person senses it immediately and that fact can prevent a leak. It is possible and relatively easy to synthesize up to 20 vol. % of ozone out from the initial oxygen. Moreover, ozone can be mixed with an efficiency of about 98% in a stream with treated water. In most cases the final substances become low toxic during the process of purification by ozone with Granular Activated Carbon (GAC) sorption. There are no auxiliary substances at the synthesis of ozone. This process is underway at the ambient temperature and under pressure, which is close to normal. Air and oxygen are renewable raw materials for further production of ozone.

The synthesis of ozone passes through only two stages. Ozone decays to safe oxygen. Analytical techniques for the determination of any concentration of ozone provide for its monitoring in real time. Usually ozone is used at a concentration of less than 22%; this is non-explosive concentration of ozone in gas. There is no danger of fire from air mixed with ozone.

Let us examine the application of the principles of green chemistry using the example of the water treatment station at the Lianozovsky dairy (PepsiCo) in Moscow.

Private sources of water supply, such as underground wells of various depths are often used for the following purposes: (1) water economy from the city's water supply system; (2) cost cutting, as water from the city's water supply system is rather expensive for industrial enterprises. However, natural waters from the city's water supply system in Moscow do not conform to the Russian norms of water quality requirements on a number of the indicators. One of them is the raised maintenance of ions of general iron (to 1–10 mg/L) (Frog 2003). Moscow Lianozovsky dairy combine (LMK) has wells where the quality of water does not comply with such kinds of requirements.

The LMK underground water from different wells has considerably different chemical characteristics. Water indicators such as chromaticity, turbidity, rigidity, and maintenance of ions of general iron do not correspond to the Russian sanitary requirements in well numbers II and III.

The research work carried out and the technology itself demonstrated that an application of the method of ozonation in combination with the GAC sorption on the granulated absorbent carbon and activated coal fiber sorbents (AF) provides full deodorization, disinfection, and decrease of concentration of ions of the total iron to the maximum permissible or much lower – to 0.03 mg/L (Tkachenko et al. 2013). Oxidation of bivalent iron in water by ozone is more effective than oxidation by air or oxygen (Table 2).

The results of R&D allowed the planning and design of the water purification station at Lianozovsky Dairy. The technological process (Tkachenko et al. 2005a) of underground water purification comprises two stages: (1) ozonation of the water (Figure 1(a)), further mixture of the almost insoluble hydroxides formed of the third valent iron on filters with two layer granular loading from fine quartz and hydroanthracite A (Figure 1(b)); (2) final sorption on the filters loaded by the GAC and activated coal fiber sorbent presented in Figure 2(a). The slime from washing the filters is dehydrated by means of the filter press (Figure 2(b)).

For ensuring the demanded concentration of ozone, we used three ozonizers with a maximum productivity of 100 g/h. The synthesis of ozone takes place in the silent barrier discharge. Also 3 m³/h of 95% of the oxygen received from air on zeolites was supplied into ozonizers. The mixing of ozone with water took place in the ozonation reactor, where bivalent iron turned into trivalent. Residual ozone decayed in the ozone destructor, filled with catalyst. The catalyst was received from byproducts of the ozonation of this water to oxygen. Later, slime containing trivalent iron was filtered by pressure filters. Then, the final water purification was carried out by the filters with absorbent carbon. Water was collected into two tanks with a capacity of 400 m³ each. Then, the water was mixed with a small dose of hypochlorite and moved to production.

Considering the high operational costs of the application of this ozonation method, we obtained the optimum dose of ozone, that is, its quantity in mg per 1 L of water, capable of oxidizing ions of total iron to the standard values.

During the adjustment and startup process, it was shown that the optimum dose of ozone makes about 0.4 g/(m³ of water). Further increase of ozone dose does not lead to an essential change of the quality of the cleared water; however, energy consumption considerably grows during the ozonation process and durability life of ozonizers also decreases.

From Figure 3 it is clear that the ozone sorption method of the water's purification allowed to improve to standard values the following characteristics of the underground water: maintenance of ions of the total iron, chromaticity, and turbidity.

About 4 tons of ferriferous slimes are annually formed at the water treatment station as the result of the return washing of the pressure filters with granular hydroanthracite. In this research, we present the ecologically safe way of processing of these ferriferous slimes with their use as raw materials in the synthesis of the catalysts suitable for cleaning of dry air-gas streams from residual amounts of ozone (Tkachenko et al. 2005b).

The main components of the ferriferous raw materials (Figure 4) are goethite and lepidocrocite. Lepidocrocite becomes iron oxide, Fe₂O₃, at a temperature of 400 °С which is confirmed (Zaloznaya & Tkachenko 2007; Tkachenko et al. 2008) by the method of X-ray phase analysis.

According to Mossbauer spectroscopy, hydroxides and oxides of iron are mostly in a fine powder condition, with a particle size of about 5–10 nm (Zaloznaya et al. 2008a) (Figure 5).

For the choice of the best ferriferous catalyst (Figure 6), the main technical characteristics were measured: average mechanical durability of a granule, porosity, specific surface, and efficiency of decomposition of ozone.

The catalyst has a chemical composition of 60 masses: % of the ferriferous raw materials and 40 masses; % special cement is mechanically strengthed and a chemically active catalyst and it can be used in industrial processes. After calcinations its durability becomes 1.8 kg/mm of diameter of one granule which is comparable with the durability of other industrial catalysts. This catalyst is characterized (Table 3) by the high values of porosity of 53% (Tkachenko et al. 2008), by the specific surface up to 200 m²/g and because it effectively destructs dry ozone (Zaloznaya et al. 2008b; Tkachenko et al. 2011a, 2011b).

The synthesis of ozone and its application in water purification is a good example of implementation of green chemistry principles in the industry. The application of ozone technology corresponds to 12 out of the 12 principles of green chemistry. Byproducts can be effectively used. At the present time, the water treatment station has cleared about 8,000,000 m³ of the underground water. This application of technology of ozonation on the basis of the principles of green chemistry led to essential cost cutting for Lianozovsky Dairy as it avoided consumption of the water from the city.