2.4.5. Disinfection and output of cleared sewer water
Disinfection of sewer water is conducted with the aim of pathogenic bacteria destroying (those that cause diseases). More often the disinfection is carried out by gaseous chlorine or by substances that contain active chlorine – chloral lime, hypochlorite, etc. Dose of chlorine for disinfection of sewer water after the mechanical clearing accounts 10mg/l, after total biological clearing – 3mg/l. In every certain case these doses should be accurate in the process of exploitation with such estimation that amount of surplus chlorine in disinfected water after the contact doesn’t exceed 1.5mg/l.
Installation for the disinfection of sewer water by chlorine consists of chlorination plant, mixers and contact reservoirs. In chloration plant the chlorator or other equipment is installed for getting of chloral water. For mixing of chloral water with sewer one the mixers of any type are used. Contact reservoirs for providing the necessary bactericidal effect take into account the 30-minutes contact of water with chlorine. Reservoirs are designed as primary settling structures and sediment from them is directed to mule sites.
Outlets are intended for mixing of cleared and disinfected water with water from reservoirs. Construction of outlet has to ensure good mixing of sewer water with water from reservoir that allows better using of self-clearing capacity of the last.
Outlets can be concentrated when sewer water is thrown through one hole, and diffusing ones when there are several outlet holes. There are coast and riverbed outlets.
Choosing of outlet construction and place of its location are determined by technical-economic calculations.
2.4.6. Methods and structures for treatment, dehydration, disinfection and utilization of sediments of sewer water
At the sewerage purifying stations the considerable amount of sediments is created. They fall out in primary settling structures and are also obtained during biological clearing of sewer water in the form of biological layer after bio-filters or the surplus active mule after aero tanks.
Sediment that is removed from primary settling tanks is named “raw”. It has grey color and contains 92-96% of water. Its volume weight accounts from 1.004 to 1.01 t/m3. In content of domestic sewer water sediment there are up to 80% of organic substances. Percentage of organic substances content in sediments of domestic sever water depending on admixtures of industrial sewer water can
range from 65 to 85%. Organic part of the sediment nearly on 80% is from carbohydrates, fat substances and proteins. In sediment there are up to 3.5% of nitrogen, 1.4% of phosphorus and 0.2% of calium that characterize it as the valuable organic fertilizer.
Raw sediment produces the danger in sanitary sphere, as it can fester, emit gas, create favorable conditions for development of bacteria and at the same time of creating the disease ones and also contain the ovum of gilments. That’s why before the utilization the sediment should be stabilized and rendered innocuous. The aerobic fermentation with usage of aerobic bacteria (aerobic stabilizations) and aerobic fermentation with anaerobic bacteria (methane tanks, septics, double row settling tanks) are used for this. During the anaerobic fermentation the disintegration of organic substances of sediment takes place in two phases.
The first phase is characterized by creation of considerable amount of fat acids (formic, acetic, oily). Besides this, in this phase carbonic acid, alcohol, amino acid, ammonia, carbon disulphide are created. Active reaction of the pH medium less than 7 that’s why the first phase has the name of “pure” fermentation. It is carried out by anaerobic bacteria of yeast type, oily-acid, propionic, butyl bacteria. In the result of acid fermentation the sediment almost doesn’t decrease in the volume, badly fall out in sediment, has unpleasant smell and can decay.
The second phase is characterized by the destructions created at the first phase with emission of carbonic acid, methane and hydrogen and carbon oxide in small amounts. Active reaction pH takes 7-8, that’s why this phase is named acid one, or of methane fermentation. Contaminants of the second phase are the methane creating anaerobic bacteria. Fermented or mature sediment after the second phase of fermentation is dried at mule sites. It has black color because of content of sulfide of iron and rubber substances; for mature sediment is characteral the pure smell of sealing wax or asphalt.
Content of organic substances in fermented sediment decreases from 80-75% to 50%. Final products of fat disintegration are the acid and methane, thus these components of organic part of sediment are supplied with gas. During disintegration of proteins only the part of created products transfer onto the gas, the rest are connected between each other or with other components and remain in solution.
Depending on local conditions, power of station and scheme of clearing for fermentation of organic sediment are used septics, double row settling tanks, methane tanks and other structures.
For clearing of sewer water and treatment of sediment at the daily expends of sewer water up to 25m3/day it’s recommended to use septics, from 25 till 10000 m3/day the double row settling tanks are used. At higher power the methane tanks or aerobic stabilizators are used.
Humidity of sediment of sewer water from the primary settling tanks accounts 93.8-95.0%, from the secondary ones: after aero tanks – 99.2-99.7%, after bio-filters – 96-96.5%.
Septics are the rectangular of round in the plan flowing reservoirs, in which clearance of sewer water and fermentation of sediment take place (fig. 2.37). Time of sewer water presence in septic – 1-3 days, and time of sediment fermentation, that fallen out - 6-12 months.
