Sludge Reduction and Value-Added Organic Products - Paper Example

CHAPTER 1

INTRODUCTION1.1 Problem statement

1.1.1 Sewage sludge treatment

Sewage sludge is an inevitable by-product during the process of wastewater treatment in wastewater treatment plants (WWTPs), and it contains two basic components: raw primary sludge and secondary sludge. With the increasingly urbanized population, the quantity of wastewater sludge is anticipated to grow significantly. And in general, the more advanced the technology applied into wastewater treatment, the more sewage sludge will produce. According to a sludge report to HKSAR Government from HKEPD (Hong Kong Environmental Protection Department), the quantity of sewage sludge produced in Hong Kong is expected to grow year by year, in 2021, the STW sludge will be up to 4.8 million tons (dry weight). Not only in Hong Kong, the total amount of sewage sludge produced in the Europe is anticipated to be 12 million tons by 2020 

Besides the quantity problem, the precarious substances which sewage sludge contains such as Persistent Organic Pollutants (POPs) would bring social and ecological problems. Some organic pollutants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) (Stevens, Northcott et al. 2003, Katsoyiannis and Samara 2004) may remain in the sludge for several months to years. Meanwhile, due to it contains numerous bacteria, protozoa, viruses and parasitic worms, if without appropriate treatment, the residual sludge will cause many pathogen risks, for example, cytomegalovirus (CMV) is discovered in 60-90% of the adult citizens in the United States which specifically poisons humans health (Lewis and Gattie 2002). Therefore, the management of sewage sludge is becoming a more and more emergent issue.

1.1.2 energy balance analysis

Biomass is often produced by aerobic digestion of organic substances such as sewage sludge, animal waste products, and municipal solids. Sustainable energy carrier such as biomass is an essential source of heat and electricity because it is composed of 55 to 75%, 24 to 45%, 0 to 5%, 0 to 1 %, 0 to 1%, and 0 to 2% of methane, carbon dioxide, nitrogen, hydrogen, hydrogen sulphide, and oxygen respectively (Saetea & Tippayawong, 2013). Moreover, sewage treatment is the process of managing the disposal of the sludge and produce. Majorly the sludge treatment focuses on the reduction of water since the slurry is mostly liquid. By removing the water, the weight and volume of the sludge reduce (EPA, 2003). There are various methods of treating sewage sludge, for example, biosolids, thickening, dewatering, side-stream treatment technologies, phosphorus recovery, digestion, composting, and incineration among others (Bachmann, 2015). The balancing analysis will focus on composting since they are conducive to rural setups. Also, it will analyse digestion and thermal hydrolysis as platforms that promote urban location sewage treatment.

Compositing sewage treatment

Compositing is an aerobic method that consists of mixing sewage sludge to agricultural by-product, for example, straws and woodchips (Anders, 2005). Moreover, the plants contain oxygen, bacteria digestion and energy digestion retrieved from the plant materials. Majorly, the three components are used to terminate the diseases cause microorganism and parasites. Also, the process uses a uniform mechanism of pathogen killing temperature. The method uses an insulating blanket placed as a layer to generate high temperatures to reduce the activities of pathogens.

Thermal Hydrolysis

Thermal hydrolysis is one of the most used techniques implemented to process biodegradability of the sludge. Majorly, the process relies on anaerobic digestion as a pretreatment stage (Menco, 2012). Also, it dissolves and disintegrates the sludge by using pressure and temperature. The products are then introduced to dewatering reactor chamber where the products will be in direct contact with saturated steam hydrolyzers responsibility is to change the internal structure of the compound (Menco, 2012). The process reduced the sludge viscosity, and increases its biodegradability; thus, producing biogas.

1.2 Objectives of study

The main purpose of this study is to investigate the possibility of applying filamentous fungus for the treatment of thermal hydrolyzed sewage sludge. Laboratory work was conducted on the study of optimum treatment condition of thermal hydrolysis of CEPT sludge and value-added organic products detection by cultivating filamentous fungus with thermal hydrolyzed CEPT sludge liquor.

In general, for this study, we have 2 main objectives:

Effectively extract organics from sewage sludge by thermal hydrolysis and therefore decrease organic pollutant in the sludge. In this part, we will do experiments to study the optimum treatment condition to obtain the comparatively best biodegradable organic extraction and characterize the soluble organics in extracted liquor.

Use the organics extracted from sewage sludge as carbon substrate for fungal fermentation to generate value-added organic metabolites and reproduce fungal hyphae as organic material. In this part we will screen the adaptive fungus strain in extracted liquor from sewage sludge first, and extract and enrich organic metabolites.

Scope of this study

Organization of the thesis

Chapter = 2 \* ROMAN II Literature review

2.1 Composition of sewage sludge

After a brief introduction of the importance of sewage sludge treatment, it is essential to look at the original composition of the sludge to figure out why if without suitable treatment the sludge will damage our society and what can we do to process this problem. As a not that precise guide, this composition is mainly characterized by six groups of components: (1) non-hazardous organic carbon compounds (nearly 60% on a dry basis), for a large portion from biological origin; (2) phosphorous- and nitrogen- containing components; (3) toxic inorganic and organic pollutants for example, heavy metals, such as Zn, Hg, Pb ,Cu, Ni, Cd and As which concentration vary from more than 1000 ppm to less than 1ppm and dioxins, polychlorinated biphenyls (PCBs), pesticides, etc.; (4) pathogens and some other microbiological pollutants; (5) inorganic compounds, such as silicates, aluminates and calcium- and magnesium- containing compounds (6)water, varying from a percentages to more than 95% (Rulkens 2008).

2.2 Sewage sludge treatment and disposal

Treatment and disposal of sewage sludge are the primary factors used in designing and operating a treatment plant. Majorly, sewage sludge is treated before finally disposed of because it is a health hazard. At the same time, it is treated to reduce the volume and stability of the organic substances. Moreover, stable sludge is environment-friendly and lacks offensive odour. The treatment process covers the digestion and dewatering of the muck. Besides, it follows through a series of stages such as thickening, metabolism, and dewatering.

Thickening Stage: It is the Thickening is the first step to sludge treatment because it is not practical to touch handle wet thin waste products because it is a health hazard. The treatment process takes place in a Gravity Thickener. During the process, sludge is suspended in water to help reduce its volume by a half the original weight. In addition, in case the plant lacks the gravity thickener, dissolved air flotation can be used. The bubbles present in the chamber will carry the solids to the top layer. Once at the top, they thicken in sludge firms.

In fact, sewage sludge treatment and disposal from wastewater treatment plants (WWTPs) accounts for 60% of the total cost of wastewater treatment (Tomei, Rita et al. 2011). Nowadays, with the consideration of energy recovery and reuse of valuable products from sludge, many cities choose digestion as a treatment of sewage sludge, and the most common treatment options include anaerobic digestion, aerobic digestion and composting (Liew, Kassim et al. 2015).

Anaerobic digestion of sludge has long been a popular and well-proven route for sludge treatment due to its ability to produce biogas (Park, Thring et al. 2011). However, the inlet feed to anaerobic digesters, which is waste activated sludge (WAS), is well-known to be difficult to dewater (Chang, Liu et al. 2001). Moreover, WAS is biological in nature and contains large proportions of bacterial cells, of which, only half are readily biodegraded in the anaerobic digester (Hii, Baroutian et al. 2014) and it required long time (up to 30 days) for the process.

Aerobic digestion, though with its advantage of low capital cost, due to the extra oxygen needed to add into the process, it will cost a lot for operating. And with the increase of population density, there will be low land availability and during the process of composting, the organic or metal contaminants contained in the sludge will cause health risks to people.

Incineration and landfilling are the common methods for the disposal of excessive sludge (Eshet, Ayalon et al. 2005), landfilling is associated mostly with the external cost of pollutants such as landfill gas (CO2 and CH4) which contributes to global warming, and lea...