sludge refers to solid, semi-solid and liquid wastes produced in the process of sewage treatment. sludge treatment is the process of sludge volume reduction and stabilization by reducing sludge moisture content. the process usually consists of sludge concentration, stabilization and dewatering units. anaerobic digestion and lime stabilization (deep dewatering) are commonly used for sludge stabilization. with the decrease of sludge water content, sludge treatment will produce a large amount of sewage, namely sludge water. sludge water mainly consists of supernatant from concentration tank, supernatant from anaerobic digestion tank and filtrate from dewatering workshop. sludge water has the characteristics of small volume (water volume is only 1%~2% of the influent of sewage treatment plant), high pollutant concentration and high temperature (about 30 c). it accounts for 5%~20%, 10%~80% and 10%~50% of the cod, nitrogen and phosphorus loads of the whole plant, respectively. the direct return of sludge water to the inlet will not only cause repetitive treatment of pollutants, but also make the effluent difficult to reach the standard [6,7]. therefore, it is important to clarify the characteristics of sludge water quality and develop its treatment technology for the stable operation of sewage treatment plants.
1 sludge water source and water quality characteristics
the sludge water quality of different sewage treatment plants varies greatly due to different sewage treatment processes, sludge sources (primary sedimentation tank sludge, excess sludge, etc.) and sludge treatment processes.
the concentration of water suspended solids (ss) in digested sludge varies greatly, which is related to the way of sludge discharging by anaerobic digestion. the ss of digestive supernatant is low, and if it is not discharged, the ss of sludge water can reach 22g/l. the concentration of nitrogen pollutants in sludge water is high, and the dissolved nitrogen mainly exists in the form of ammonia nitrogen. in the process of mechanical concentration and dewatering, interstitial water is extruded and some extracellular polymers are dissolved, so the concentration of organic pollutants, nitrogen and phosphorus is higher. at the same time, in the anaerobic environment of concentration, dehydration and anaerobic digestion, phosphorus in sludge is released again, so the phosphorus content of sludge water is very high, and most of it exists in the form of phosphate. deep dewatering sludge water has the characteristics of high ph, high calcium and low phosphorus, which is mainly due to the addition of lime and fecl3 during sludge conditioning stage, resulting in the reaction of phosphate with ca2 and fe3 to form precipitation and remove.
2 sludge water treatment technology
2.1 physicochemical treatment technology of sludge water
2.1.1 ss removal technology
as shown in table 1, ss concentration in sludge water is high and its fluctuation range is wide. chemical oxygen demand (cod), total nitrogen (tn) and total phosphorus (tp) mainly exist in granular and colloidal state. ss removal is the key to reduce pollutants in sludge water. liu fanjia et al. studied the treatment effect of coagulation sedimentation on supernatant and dewatering filtrate of sludge concentrator. it was found that polyacrylamide (pam) produced larger flocs than polyaluminium chloride (pac) and was easier to precipitate. the removal rate of ss, cod and bod5 by pam coagulation was more than 90%. zhou zhenwei compared the pretreatment effects of pac, pfc and pam on concentrated dewatered sludge water. it was found that adding pac would deteriorate sludge settling performance. adding pfc and pam can improve sludge water settling performance.
removal of ammonia nitrogen by 2.1.2 stripping method
when ph > 10, ammonia nitrogen mainly exists in nh3, which can be removed by stripping method. gu2tin et al. studied the effects of ph, temperature and gas flow rate on the removal of ammonia nitrogen from anaerobic digestion supernatant by blowing. the removal rates of ammonia nitrogen and total nitrogen were 92.2% and 88.3%. the stripping method has better effect on ammonia nitrogen removal of deep dewatered sludge water with high ph and alkalinity, and can simultaneously reduce sludge water ph, which is helpful for its subsequent treatment. calcium carbonate scaling should be taken into account when deep dewatering sludge water is blown off.
2.2 biological denitrification technology of sludge water
research by the netherlands applied water research foundation shows that the treatment of sludge water with high ammonia nitrogen (see table 1) alone is of great significance for upgrading wastewater treatment plants and saving treatment costs compared with reflux to wastewater treatment processes.
2.2.1 traditional denitrification technology
as shown in table 1, the bod5/tn ratio of sludge water is below 4, which can not meet the requirement of carbon source for conventional nitrification-denitrification. when yu lifang and others used sequencing batch reactor (sbr) to treat sludge anaerobic digestion liquid, the removal rate of ammonia nitrogen could reach 99.3%, but the removal rate of tn was only 38.1% due to insufficient carbon source. the removal rates of ammonia nitrogen and tkn were 99.3% and 97.4% respectively, but the removal rate of tn was only 24.3%. liu fanjia et al. treated concentrated dewatered sludge water after coagulation and sedimentation with membrane-bioreactor (mbr) enhanced denitrification and phosphorus removal pilot-scale system. the effluent cod, bod5, ammonia nitrogen, tn and tp concentrations could reach 70.8 mg/l, 8.7 mg/l, 15.1 mg/l, 29.7 mg/l and 0.38 mg/l, respectively.
in addition to the lack of carbon sources, the traditional concentrated dewatered sludge water often faces the problem of insufficient alkalinity. hu et al. found that adding sodium bicarbonate alkalinity to mbr of sludge water treatment could increase cod removal and nitrification efficiency by 14.6% and 38.3%, respectively, and could alleviate membrane fouling and enrich nitrifying bacteria. in order to reduce the cost, the deep dewatered sludge water with high ph can also be used as a source of alkalinity and carbon to form an economical and effective sludge water denitrification model.
2.2.2 enhanced nitrification by biological addition of enriched nitrifying bacteria
(1) enhanced nitrification by inoculation. enhanced nitrification by inoculation is the introduction of microorganisms with specific functions into biological treatment systems to improve the removal efficiency of target pollutants in the original system. sludge water was treated separately by bio-enhancement technology developed by royal swedish institute of technology. sludge water was treated by side-flow reactor. nitrifying bacteria enriched and cultured were continuously added to the mainstream sewage treatment system at a certain concentration to enhance their nitrification capacity and achieve short sludge age nitrification. compared with traditional activated sludge process, inoculation intensification can not only improve the removal efficiency of refractory cod and ammonia nitrogen, but also improve sludge performance.
(2) innitri process. innitri process is the first one to use sludge digestion liquid to cultivate nitrifying bacteria and add them to the main sewage treatment system for intensified nitrification. innitri used a separate reactor (3% of the volume of the aeration tank in the main system) to treat sludge digestion (30-35 c, 300-900 mg/l ammonia nitrogen) and effluent from a part of the primary sedimentation tank. the nitrifying sludge age was only 7-10 days at 10 c, which was significantly lower than that of the traditional denitrification process for 13-18 days. innitri process can reduce investment in upgrading and renovation of wastewater treatment plants by 10%~15%. however, because nitrification only takes place in the innitri process, a large amount of alkalinity needs to be increased, and the high concentration of no3-n in the effluent is not conducive to the operation of the sedimentation tank and the removal of phosphorus from the system, so it has not been reported in practical engineering application.
(3) babe process. in view of the characteristics of sludge water with high ammonia nitrogen, salem et al. [5] developed a reactor to enrich nitrifying bacteria in sludge reflux pipeline, thus forming a babe process of sidestream enrichment of nitrifying bacteria and mainstream enhancement of nitrification capacity. compared with innitri process, babe process has secondary clarifier sludge reflux, which can not only utilize the alkalinity of reflux sludge, but also ensure the consistency between enriched and cultured nitrifying bacteria and dominant nitrifying bacteria in the main system in population, ecological environment and other aspects. moreover, nitrifying bacteria are included in biological flocs and are not easy to be mainstream treatment system. protozoa prey in tongzhong. in addition, a smaller amount of sludge and a shorter residence time can also keep the reactor at a higher temperature, thus achieving a higher nitrification rate. after using babe technology in garmerwolde wastewater treatment plant in netherlands, ammonia nitrogen in effluent decreased from 9.9 to 13.3 mg/l to 5.2 mg/l. of course, there are also reports that external culture will form special nitrifying bacteria, which may not be able to adapt to the environment of the main sewage treatment system.
2.2.3 short-cut nitrification and denitrification and anaerobic ammonia oxidation
due to the low c/n ratio of sludge water, the short-cut nitrification and denitrification process firstly oxidizes nh4 to no2-, under aerobic conditions, and then denitrifies no2-to n2 under anoxic conditions. this process can save 25% oxygen supply and 40% carbon source compared with the traditional process. under the conditions of short hrt and 30-40 c, nitrite bacteria can become dominant species through population competition. short-cut nitrification and denitrification can be achieved by controlling the higher temperature of sludge digestion liquid and sludge age. sharon process has been successfully applied to sludge digestion treatment in dokhaven and u-trecht wastewater treatment plants in the netherlands. the denitrification rate is higher than 90%.
3 conclusions and prospects
(1) sludge water mainly comes from sludge concentration, digestion and dewatering in sewage treatment plants. the characteristics of sludge water from concentrated dewatering and anaerobic digestion are high ss content, high nitrogen and phosphorus content, relatively insufficient nitrogen, phosphorus and carbon sources and alkalinity.
(2) chemical coagulation can effectively remove s s and total phosphorus from sludge water. it is the main research trend in the future to determine suitable coagulants and low-cost dephosphorizers according to the quality of sludge water.
(3) the application of high ammonia nitrogen in sludge water to enrich nitrifying bacteria in sidestream and strengthen nitrification in mainstream sewage treatment system has a good application prospect in upgrading and renovation of sewage treatment plants. sharon and anam-mox processes can effectively solve the problem of insufficient carbon source in sludge water denitrification.
source: polaris environmental protection network
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