Effects and model of alkaline waste activated sludge treatment

doi:10.1016/j.biortech.2007.09.019

Bioresource Technology

Volume 99, Issue 11, July 2008, Pages 5140-5144

https://doi.org/10.1016/j.biortech.2007.09.019 Get rights and content

Abstract

Due to lack of information about alkaline sludge treatment with high dose at ambient temperature, alkaline sludge treatment was investigated to know the effects of different doses (0–0.5 mol/L) of sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)₂] at 0–40 °C. Results showed that NaOH was more suitable than Ca(OH)₂ for sludge disintegration. For NaOH treatment, most efficient dose was about 0.05 mol/L (0.16 g/g dry solid), and 60–71% solubilization of organic matters was achieved in first 30 min during its treatment of 24 h. The process is described by a transmutative power function model, and relation between reaction rate constant and temperature are according with Arrhenius equation. Low dose NaOH treatment (<0.2 mol/L) deteriorated sludge dewatering ability obviously, while the ability was recovered gradually with the increase of NaOH dose. For Ca(OH)₂ treatment, the disintegrated floc fragments and soluble organic polymers can be re-flocculated with the help of calcium cations. Consequently sludge disintegration effect was counteracted and dewatering ability improved.

Introduction

Waste activated sludge (WAS) is the main by-product of wastewater treatment process, and it is about 0.5–1% of total influent water. In China 20–50% of capital and operation cost of wastewater treatment plants are spent on WAS treatment and disposal. So sludge reduction is very important for its economical treatment. Sludge disintegration, a pretreatment method of dewatering or anaerobic digestion, has been studied extensively and proved in improvement of sludge reduction processes. Sludge disintegration methods include mechanical, thermal, chemical and biological treatments. Comparing with other methods alkaline treatment has the several advantages, i.e. simple manufacturing of device, easy to operate and high efficiency (Weemaes and Verstraete, 1998). Alkaline sludge treatment can disrupt flocs and cells, release inner organic matters and accelerate sludge hydrolysis, and consequently improve the performance of succedent anaerobic digestion (Novelli et al., 1995, APHA, 1995, Lin et al., 1997, Kim et al., 2003, Cassini et al., 2006, APHA, 1995, Lin et al., 1997, Kim et al., 2003, Cassini et al., 2006). Besides this, alkaline sludge treatment can also release the water held inside floc and cell structure, which can not be removed by conventional dewatering processes. Therefore, alkaline treatment can improve sludge dewatering ability (Erdincler and Vesilind, 2000, Neyens et al., 2003).

Alkaline sludge treatment depends on dissolution or destruction of floc structure and cell wall by hydroxy radicle. Extracellular polymer substances (EPS) hold on sludge particles together to form flocs. EPS include protein, humic substances, polysaccharid, lipid and nucleic acid (Dignac et al., 1998). An extreme of high pH causes protein to loose their natural shapes, saponification of lipid and hydrolysis of RNA (Zhang, 2002). Strong alkali solubilize gels not only because of chemical degradation but also ionization of the hydroxyl groups (–OH → –O⁻), which leads to extensive swelling and subsequent solubilization (Neyens et al., 2004). After destruction of EPS and gels, cells are exposed to environment with extremes of pH thereby cannot keep the appropriate turgor pressure. Due to aforementioned reason and saponification of lipid, cells are disrupted and the inner matters are released. Therefore, alkaline treatment can solubilize sludge and release inner water.

Most of the investigations exhibit increase of soluble chemical oxygen demand (SCOD) or decrease of volatile suspended solid (VSS) especially during low dose (<0.1 mol/L) alkaline treatment or its combining treatment with thermal treatment (Lin et al., 1997, Erdincler and Vesilind, 2000, Navia et al., 2002, Neyens et al., 2003, Vlyssides and Karlis, 2004, Cassini et al., 2006). But low dose alkali generally acts as auxiliaries in thermal hydrolysis (50–200 °C), and less information is available on high dose alkaline treatment at ambient temperature range, especially its effect on sludge dewatering ability. The aim of this study is to fill up some gaps in the research. We examined both sludge disintegration effect and dewatering ability during alkaline treatment with low and high dose of sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)₂] at ambient temperature range (0–40 °C). Based on the analyses, this study can provide more insights into alkaline sludge treatment and establish an experimental model to describe the process.

Section snippets

Methods

Sludge samples were collected from the outlet of aerobic and thickening tanks of a local full-scale municipal wastewater treatment plant and were stored at 4 °C before usage. The process of waste water treatment plant is anaerobic–anoxic–aerobic. The sludge samples had a water content of 98%, pH of 6.9, SCOD of 275 mg/L, suspended solid (SS) of 12,788 mg/L and volatile suspended solid (VSS) of 9630 mg/L. To analyze the effect of sludge concentration on alkaline treatment, sludge concentration was

Solubilization of organic matters

The effects of NaOH dose on sludge disintegration are presented by SCOD change (Fig. 1). The SCOD increased with NaOH dose, and treatment was the most efficient when dose was about 0.05 mol/L (0.16 g/g DS). It indicated that NaOH was excess. In fact, sludge pH value was still over 12 after 0.05 mol/L NaOH treatment of 30 min. Consequently the quantity of solubilized organic matters was proportional to sludge concentration, and in this study the ratio of SCOD increment and sludge VSS was about 0.63

Conclusions

NaOH was more efficient for sludge disintegration. The process included two stages: a rapid initial phase of 0.5 h duration and subsequent slower phase. Sludge disintegration degree was decided by NaOH dose, and at the same dose the quantity of dissolved organic matters was proportional with sludge concentration. Low dose NaOH treatment deteriorated sludge dewatering ability obviously, while the ability can be restored at some degree by the treatment with high dose.

Ca(OH)₂ treatment was more

Acknowledgements

This work was supported by China National Eleven Five-Year Scientific and Technical Support Plans (No. 2006BAC02A18).

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