Nitrogen removal from reject water with primary sludge as denitrification carbon source

Nitrate addition improves hydrogen production from acidic fermentation of waste activated sludge

In this work, a low-cost alternative method (i.e., adding nitrate into WAS) to significantly enhance hydrogen production was reported. Experimental results showed that with an increase of nitrate addition from 0 to 362 mg/L, the maximal hydrogen production from acidic (pH 5.5) fermentation of WAS obviously increased from 12.6 ± 0.5 to 19.3 ± 0.9 mL per gram volatile suspended solids (VSS). The mechanism investigations illustrated more substrates were provided for subsequent hydrogen production. Although the nitrate added inhibited all the biological processes, its inhibition to the hydrogen consumption processes was much severer than that to the hydrogen production processes. The enzyme analyses on the long-term semi-continuous fermenters showed that the nitrate addition slightly inhibited the relative activities of protease, butyrate kinase, acetate kinase, CoA-transferase, and [FeFe] hydrogenase but largely suppressed the relative activities of coenzyme F₄₂₀, carbon monoxide dehydrogenase, and adenylyl sulfate reductase.

Free nitrous acid promotes hydrogen production from dark fermentation of waste activated sludge

Simultaneous sludge fermentation and nitrite removal is an effective approach to enhance nutrient removal from low carbon-wastewater. It was found in this work that the presence of nitrite largely promoted hydrogen production from acidic fermentation of waste activated sludge (WAS). The results showed that with an increase of nitrite from 0 to 250 mg/L, the maximal hydrogen yield increased from 8.5 to 15.0 mL/g VSS at pH 5.5 fermentation and 8.1-13.0 mL/g VSS at pH 6 fermentation. However, the maximal hydrogen yield from WAS fermentation at pH 8 remained almost constant (2.9-3.7 mL/g VSS) when nitrite was in the range of 0-250 mg/L. Further analyses revealed that free nitrous acid (FNA) rather than nitrite was the major contributor to the promotion of hydrogen yield. The mechanism investigations showed that FNA not only accelerated the disruption of sludge cells but also promoted the biodegradability of organics released, thereby provided more biodegradable substrates for subsequent hydrogen production. Although FNA inhibited activities of all microbes involved in the anaerobic fermentation, its inhibitions to hydrogen consumers were much severer than those to hydrolytic microorganisms and hydrogen producers. Further investigations with microbial community showed that FNA increased the abundances of hydrogen producers (e.g., Citrobacter sp.) and denitrifiers (e.g., Dechloromonas sp.), but reduced the abundances of hydrogen consumers (e.g., Clostridium_aceticum). This work demonstrated for the first time that FNA in WAS fermentation systems enhanced hydrogen production. The findings obtained expand the application field of FNA and may provide supports for sustainable operation of wastewater treatment plants.

Energy-positive nitrogen removal from reject water using a tide-type biocathode microbial electrochemical system

A tide-type biocathode microbial electrochemical system (TBMES) employing intermittent air accessible method was constructed for simultaneous carbon and nitrogen removal. The nitrification and denitrification processes occurred in cathode chamber were enhanced by raising frequency of catholyte feeding-draining process and lowering external resistance. At external resistance of 5Ω and frequency of 8cph, the TBMES removed 99.3±0.3% of COD and 57.7±1.1% of total nitrogen when treating synthetic medium with COD/N ratio of 3.0, concomitantly, a maximum power density of 10.6Wm^-3 was achieved. Comparable performances were obtained for reject water treatment with a relatively lower COD/N ratio of 2.5, which were 88.6±1.3%, 53.2±3.8% and 8.9±0.2Wm^-3 for COD removal, total nitrogen removal and maximum power density. The feeding-draining process consumed 14.3% of the total energy produced, and thus obviated energy-intensive aeration and achieved net energy output.

Free nitrous acid pretreatment of wasted activated sludge to exploit internal carbon source for enhanced denitrification

Using internal carbon source contained in waste activated sludge (WAS) is beneficial for nitrogen removal from wastewater with low carbon/nitrogen ratio, but it is usually limited by sludge disintegration. This study presented a novel strategy based on free nitrous acid (FNA) pretreatment to intensify the release of organic matters from WAS for enhanced denitrification. During FNA pretreatment, soluble chemical oxygen demand (SCOD) production kept increasing when FNA increased from 0 to 2.04 mg HNO2-N/L. Compared with untreated WAS, the internal carbon source production increased by 50% in a simultaneous fermentation and denitrification reactor fed with WAS pretreated by FNA for 24 h at 2.04 mg HNO2-N/L. This also increased denitrification efficiency by 76% and sludge reduction by 87.5%. More importantly, greenhouse gas nitrous oxide production in denitrification was alleviated since more electrons could be provided by FNA pretreated WAS.

Mechanisms of nitrite addition for simultaneous sludge fermentation/nitrite removal (SFNR)

Simultaneous sludge fermentation and nitrite removal (SFNR) was investigated as a novel sludge/wastewater treatment process with high nitrogen concentrations. The results showed that introducing nitrite improved the primary sludge (PS) fermentation system by improving the chemical oxygen demand (COD) yields and the volatile suspend solid (VSS) reduction. At a nitrite dosage of 0.2 g g SS(-1), the COD production was 1.02 g g VSS(-1) and the VSS reduction was 63.4% within 7-day fermentation, while the COD production was only 0.17 g g VSS(-1) and the VSS reduction was only 4.9% in the blank test. Nitrite contained in wastewater was removed through denitrification process in the SFNR system. The solubility of carbohydrate and protein was substantially enhanced, and their contents reached the peak once nitrite was consumed. In addition, the nutrient release and methane generation were inhibited in the SFNR system, which alleviated the environmental pollution. Unlike traditional fermentation systems, neither alkaline condition nor high free nitrite acid (FNA) concentration affected the PS fermentation in the SFNR system. Molecular weight distribution (MWD) and Live/Dead cell analysis indicated that the sludge disruption by nitrite and the consumption of soluble organic substances in sludge might play important roles in SFNR.

The feasibility of using a two-stage autotrophic nitrogen removal process to treat sewage

The feasibility of using a two-stage autotrophic nitrogen removal process to treat sewage was examined in this study. The obtained results showed that total nitrogen (TN) could be efficiently removed by 88.38% when influent TN and chemical oxygen demand (COD) were 45.87 and 44.40 mg/L, respectively. In the first stage, nitritation was instantly achieved by the bioaugmentation strategy, and can be maintained under limited oxygen condition (below 0.2mg/L). The ratio of nitrite to ammonium in the effluent of the nitritation reactor can be controlled at approximate 1.0 by adjusting aeration rate. In the second stage, anammox was realized in the upflow anaerobic sludge blanket (UASB) reactor, where the total nitrogen removal rate was 0.40 kg Nm(-3)d(-1) under limited-substrate condition. Therefore, the organic matter in sewage can be firstly concentrated in biomass which could generate biogas (energy). Then, nitrogen in sewage could be removed in a two-stage autotrophic nitrogen removal process.