Municipal sewage sludge (MSS) disposal is an urgent issue in China with the continuous growth of sewage treatment capacity. Among various disposal methods, co-processing of MSS in cement kilns has been one of the most promising disposal methods in recent years. The present situation of sewage treatment and sludge disposal, the development of co-processing MSS in a cement kiln, and main disposal routes in China were discussed in this study. The results indicated that China had paid considerable attention to the technology and released correlative policies in the past few years. There were about 35 co-processing projects built in China, all of which were limited by construction scale and pollutant emissions. Due to differences in construction methods and economic conditions, China’s co-processing projects mainly employed three routes—direct addition to a transition chamber, addition to a precalciner after direct thermal drying, and addition to a precalciner after indirect drying. Summarizing and analyzing the characteristics of MSS co-processing would facilitate its development in China and similar regions.
Thermal drying was used to reduce sludge moisture content before co-combustion in cement kilns. The characteristics of ammonia (NH3) emission during thermal drying of lime sludge (LS) were investigated in a laboratory-scale tubular dry furnace under different temperature and time conditions. As the temperature increased, the NH3 concentration increased in the temperature range 100-130°C, decreased in the temperature range 130-220°C and increased rapidly at >220°C. Emission of NH3 also increased as the lime dosage increased and stabilized at lime dosages>5%. In the first 60 min of drying experiments, 55% of the NH3 was released. NH3 accounted for about 67-72% of the change in total nitrogen caused by the release of nitrogen-containing volatile compounds (VCs) from the sludge. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy revealed that the main forms of nitrogen in sludge were amides and amines. The addition of lime (CaO) could cause conversion of N-H, N-O or C-N containing compounds to NH3 during the drying process.
In order to assess the effects of the surface hydrophilicity of supports on the biofilm formation and evaluate the performance of completely autotrophic nitrogen removal over nitrite (CANON) process in a sequencing batch biofilm reactor (SBBR), unmodified activity carbon fibers (ACFs) and ACFs hydrophilic modified by heat treatment were used as supports. CANON process was initiated in a SBBR from conventional activated sludge. An operation temperature of 32 ± 2 °C, dissolved oxygen (DO) level at 1.5 mg L⁻¹ and free ammonia (FA) concentration with 3.98-15.93 mg L⁻¹ were maintained in the SBBR. Fourier transform infrared (FT-IR) spectra and Boehm's neutralizing titration exhibited that modified ACFs had more oxygen-containing groups than unmodified ACFs. Larger biofilm growth on the modified surfaces examined by scanning electron microscopy and biofilm's total dry weight, and the biofilm on the modified surfaces were more active, compared with those on the unmodified surfaces. This study demonstrates the hydrophilic-modified ACFs have better biological affinity than unmodified ACFs. Maximal total nitrogen removal rate of 0.088 k g N m⁻³ day⁻¹ was achieved for the CANON process on day 80, indicating the CANON process was successfully started up. Apart from supports, the strategies of DO supplying and controlling FA concentration were also keys in starting up the CANON process within a shorter period.
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