Enhancing sludge biodegradability through free nitrous acid pre-treatment at low exposure time

The effectiveness of low free nitrous acid (FNA) pre-treatment times (PTs) (5 h) resulted in a decrease in MP due to a volatile solid reduction on WAS during the pretreatment. The highest FNA concentration tested (4.62 mg N-HNO/L) did not further improve MP and SMP. This study clearly shows the effectiveness of the FNA sludge pretreatment at low exposure times

Effect of free nitrous acid pre-treatment on primary sludge at low exposure times

The present study was undertaken to investigate the effect of different free nitrous acid (FNA) concentrations at low pre-treatment times (PTs) (1, 2 and 5h) and without pH control with mild agitation on primary sludge (PS) biodegradability and methane production (MP). Increasing PTs resulted in an increase in the solubility of the organic matter (around 25%), but not on cell-mortality (>75% in all the cases with FNA) and neither on methane generation. FNA pre-treatment at low PTs improve MP (around 16% at PT of 1h and 650mg N-NO2-/L). However, a similar improvement was found with mild agitation of PS without FNA at 2 and 5h. Taking into account the potential costs associated with the FNA pre-treatment, a mild agitation without FNA would be preferred to enhance MP in PS

Exploring alternatives to reduce economical costs associated with FNA pre-treatment of waste activated sludge

Recent studies have shown the effectiveness of Free Nitrous Acid (FNA) pre-treatment in enhancing sludge biodegradability and improving its methane production potential. FNA is regarded as an environmental friendly pre-treatment which can be easily applied when a source of nitrite is present in wastewater treatment plants. However, when nitrite is not available and needs to be purchased, this treatment can become less attractive due to the costs associated to nitrite. In order to overcome this possible limitation, two different strategies to optimize the use of nitrite during FNA treatment were investigated: i) Recovering NO after the pre-treatment is completed; and ii) Concentrating the sludge before FNA pre-treatment. Results show that recovering NO from the pre-treated sludge is not suitable due to the loss of soluble organic matter present in the supernatant after the pre-treatment. However, concentrating the sludge before the pre-treatment seems a good strategy to optimize the use of nitrite

Assessment of free nitrous acid pre-treatment on a mixture of primary sludge and waste activated sludge: effect of exposure time and concentration

Free nitrous acid (FNA) has been shown to enhance the biodegradability of waste activated sludge (WAS) but its effectiveness on the pre-treatment of mixed sludge is not known. This study explores the effectiveness of four different FNA concentrations (0, 2.49, 3.55, 4.62 mg N-HNO2/L) and three exposure times (2, 5, 9 h) lower than the ones reported in literature (24 h) on WAS characteristics and specific methane production (SMP). FNA pre-treatment reduced sludge cell viability below 10% in all cases after an exposure time of 5 h, increasing the solubility of the organic matter. The treated mixed sludge was used as substrate for the biochemical methane production tests to assess its SMP. Results showed a significant increase (up to 25%) on SMP when the sludge was pretreated with the lowest FNA concentration (2.49 mg N-HNO2/L) during 2 and 5 h but did not show any improvement at longer exposure times or higher FNA concentrations. (C) 2016 Elsevier Ltd. All rights reserved