【正文】 and the acidbasic balance are controlled by carbonic acid and the ionization of a kind ofalkali . The latter is collaborated by VFA , ammonia and the other strong acid and alkali .It is tested that the range of concentration of P and sulfideS are mg/L , Mg/L and mg/L correspondingly , the average values are , and mg/L , respectively. According to the previous work mentioned above , the buffering capacity of ammonia , phosphate and hydrosulphuric acid can be neglected in TPD wastewater. Then , which one is responsible for the buffering of the acidbase balance when EGSB process is applied to treat the TPD wastewater 6 How to obtain a stable pH with the range of in such a carbonic acid Pbicarbonate system 6 TA is dualorganic acid , which exists in water with two states of molecule or ion state. Further more , its solubility connected closely with pH. The ionization equilibrium is shown as follows (Editorial Board of Encyclopedia of Chemical Industry , 1990) : VFA and UFA in the mixed liquorIt is known that the anaerobic process involves two major groups of bacteria that degrade organic substrate in two major ways. In the first stage ,揳cidform攂acteria hydrolyze and degrade the plex organic substrates to volatile fatty acids (VFA ) . In the second stage , VFAs are utilized by methanogenic bacterial and methane gas is produced. The two processes occur simultaneously and process stability depends on maintenance of a delicate biochemical balance between the two major phases for organic substrates.Anaerobic process instability is usually indicated by a rapid increase in the concentration of VFAs with a subsequent decrease in methane gas production. There are many factors associated with instability or process failure , for example , insufficient acclimation of the methane formers to new substrates , rapid temperature or pH fluctuations. From the results by Kroeker et al . (Andrews , 1969 。 Kroeker , 1979) , anaerobic digestion toxicities related more directly to excessive concentration of un2ionized volatile acids (UFAs) , although they probably indirectly related to excessive free ammonia concentrations. What is more , the toxicities have a close correlation with pH due to the reason that UFA is easier than other pounds to penetrate through cell membrane. As soon as microbe assimilates the UFA , pH in the cell depresses rapidly and metabolic rate of the microbe reduces. It is still acceptable that the digester toxicity was caused by UFA in a concentration above 30 to 60 mg/L as acetic acid. Balance of VFA and alkalinityThe typical variation of alkalinity and VFA in EGSB reactor is shown in Fig. 11. The alkalinity concentration of anoxic effluent was approximately equal to that of TPD wastewater in a range of 900 6000 mg/L. The rebound range was about 100 mg/L. Total alkalinity concentration increased about 250 mg/L to 1150 6250 mg/L after treatment of EGSB. Influent ( TPD wastewater ) VFA 90 633 mg/L increased to 197 636 mg/L after anoxic treatment , while the effluent VFA from EGSB reactor was mg/L with the removal efficiency % %. The increase oftotal alkalinity in EGSB reactor could be presumed to relate with the consumption of VFA.The alkalinity is buffering capacity to resist the VFA in anaerobic system. The calculation mentioned above indicated that VFA of the wastewater hardly inhibited anaerobic process. The concentration of alkalinity was high enough for buffering the VFA in the mixed liquor. Lesilie (Lesilie , 1989) took for the ratio of VFA to total alkalinity to evaluate the buffering capacity of anaerobic system(Leslie , 1989) . The total alkalinity only descended lightly during anoxic treatment and ascended lightly in EGSB reactor. The ration of VFA to totalalkalinity was always low than 0141 The ratio of VFA to total alkalinity associated with the stability of anaerobic system is shown in Table 3 (Leslie , 1989 ) . Obviously , the variation of VFA concentration only caused little change of pH when TPD wastewater was treated in EGSB reactor. The buffering capacity of the mixed liquor was sufficient for EGSB process.4 ConclusionsTA , the dualorganic acid of TA was responsible for the most of the alkalinity of the wastewater and provided sufficient buffering capacity for the acidbase balance when EGSB process was applied to treat the TPD wastewater. The EGSB reactor抯 performance improved and became more stable along with the sludge granulating.Nomenclature:BOD5 : biochemical oxygen demand for 5 days , mg/L 。 SS: suspended solid 。 COD : chemical oxygen demand , mg/L 。 VFA : volatile fatty acid 。 COL : color 。 UFA : unionized volatile acid 。 EGSB : expanded granular sludge bed 。 Vg : biogas production rate , m3/( kg COD) 。 HRT: hydraulic retention time , h 。 T : temperature , ℃。 K: ionization constant 。 TA : trephthalic acid 。 MLSS: mxed liquor suspended solid , g/L 。 ηCOD : eficiency of COD removal , %。 PVA : polyvinyl alcohol .References:[1 ] Andrews J F , 19691 Dynamic model of the anaerobic digestion process [ J ] .Sanitary Engineering Division ASCE , 95 (SA1) : 95 6161.[2 ]Capri M G, Marais G V R , 19751 pH adjustment in anaerobic digestion [ J ] .Water Research , 9(3) : 307 6131.[3 ]Editorial Board of Encyclopedia of Chemical Industry , 19901 Encyclopedia ofchemical industry[M] . Vol 11 Beijing.[4 ]Chemical Industry Press. 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