Biomethane potential test for rapid evaluation of anaerobic digestion of sewage sludge from multiple materials for a proposed large scale digester

Biomethane potential test for rapid evaluation of anaerobic digestion of sewage sludge from multiple materials for a proposed large-scale digester PART I: INTRODUCTION1.1 Research ration

Biomethane potential test for rapid evaluation of anaerobic digestion

of sewage sludge from multiple materials for a proposed large-scale digester

PART I: INTRODUCTION1.1 Research rationale

In recent years, there are several techniques for the treatment and management

of sewage sludge, including landfill, incineration, composting, and anaerobic digestion (AD) process Among them, AD is the most commonly used technique since biogas, which is a valuable form of bio-energy, can be extracted from

sewage waste

AD is a process by which organic materials are naturally broken down into

biogas and bio-fertilizer In this process involving several sequential biochemical stages, many different microorganisms participate in a complex web of

interacting processes which result in the decomposition of complex organic compounds as carbohydrates, fats and proteins to the final products as methane and carbon dioxide This process occurs naturally in many environments with limited access to oxygen, for example in bogs and marshes, rice paddies and in the stomach of ruminants, such as cows

Besides, it happens in the absent of oxygen naturally, or in sealed, free-oxygen tanks called anaerobic digesters AD of solid organic waste such as bio-waste, sludge, cattle manure, energy crops, and other biomasses, for bio-energy

production is widely applied technologies The production of biogas in AD offers

several advantages over the other alternatives These include biogas production, nutrient recovery, and reduction of waste organic content and pathogen agents.Sewage sludge can be described as a byproduct mixture of solid and water from wastewater treatment (CIWEM, 1995) By applying several different treatment processes, the resulting sewage sludge types extremely differ in their

characteristics Constituents of sewage sludge regarding to carbonhydrate,

proteins, lipids are highly depended on their origin The presence of significant concentration of nitrogen, phosphorus, and potassium in sewage sludge make it possible for fertilizing soil because these elements are essential for plant growth

AD instability is resulted from the fluctuation in organic loading rate,

heterogeneity of waste or excessive inhibitors Towards improving AD

performance in biogas production and accelerating the microbial activities for higher quality of bio-solids, several environmental conditions should be

meticulously controlled Additionally, various studied have demonstrated that hydrolysis phase is a rate-limiting stage, and seriously impacts on the

performance of AD

At present, end-users in Vietnam, often have difficulties in controlling the

technology efficiently, due to poor management competence (Jiang et al., 2011) This leads to production being inadequate in periods of high demand in low temperature regions during winter, and excessive during periods of high

temperature and high production of excreta (Cu et al., 2012) There is thus a need

to improve knowledge about biogas production potential using local biomass, in

order to develop digesters adapted to the local environment and individual management schemes, thus ensuring production of the gas needed for cooking, heating and light (Vu et al., 2007; Cu et al., 2012) Hence, there is an associated need to review, develop and validate methods to assess biogas production which can be used in laboratories with limited access to analytical instruments

Research carried out at laboratories in regions with limited access to high-tech instruments must be of international standard, so as to ensure useful results and contribute to progress in development of the technology

chromatography (GC) and by absorption of CO2 in alkaline liquid (7M NaOH) The objective is to recognize substrates with a high methane production per unit mass in lab-scale with limited access to analytical equipment, which will lead to

an economic digester design in future

1.3 Research questions

This study is operated to investigate these following issues:

o How much gas can be expected to be produced from the substrates?

o What is methane potential of the substrates?

1.4 Limitation

The increasing demand of renewable source of energy and quality of bio-solids has determined as a great deal to formulate the feasible treatment processes applied in WWTPs In addition to sewage sludge stabilization, AD has been

known to produce biogas, which is renewable fuel Using organic materials is expected to enhance the efficiency of anaerobic digesters Furthermore, a more comprehensive understanding of key physiochemical properties of the substrate,operational conditions, and biogas potential is of great necessary prior to any large-scale opperations

The BMP assay is designed to provide ideal anaerobic conditions and prevent anyform of biochemical inhibition To ensure this, three important conditions should

be met throughout the BMP assay (Labatut, et al, 2010): (1) appropriate

microbial community, enzyme pool, and nutrients are present; (2) environmentalconditions are optimal; and (3) substrate and intermediate product

concentrations are well below inhibitory/toxic levels BMP results should be limited to a relative interpretation of the substrate’s methane potential, and not for an absolute estimation of daily biomethane yields or the overall performance and stability of large-scale digesters, it is best suited when used to elucidate whattypes of substrates, from an array of potential substrates, have the highest

biomethane potential

PART II: LITERATURE REVIEW

This chapter provides an overview of the current knowledge regarding

bio-methane potential test, including anaerobic digestion of sludge sewage and other organic waste materials The AD process is firstly presented and discussed This

is followed by a comprehensive review of CH4 production by anaerobically digesting sewage sludge with other substrates

2.1 Sewage sludge

In the effort of improve effluent quality, waste water treatments (WWTs) are built and upgraded While these plants can produce high effluent quality, sludge disposal remains underlying issues These include the expensive cost of sludge treatment, which makes up more than 50% of total WWTs cost (Rulkens, W., 2007), and potential risks associated with the sludge disposal for the

environment and human health

Sewage sludge is a mixture of solids and semi-solid removed from the liquid stream of WWPs A more restricted definition is “a residual solid from sewage

plants treating domestic and urban waste water and from other sewage plants treating waste water of a composition similar to domestic and urban waste water”

2.1.1 Type of sewage sludge

To assess options for sludge treatment and disposal, it is necessary to investigate different kind of sludge and origins A typical sewage treatment plants includes primary, secondary, and tertiary processes (Fytili, D., et al, 2008)

Primary sludge is collected from primary treatment process containing high totalsolids (TS) content The characteristics of primary sludge vary considerably depending on the initial compositions of wastewater, the efficiency of primary sedimentation and the usage of chemicals in sedimentation (Guyer, J.P., 2011)) Primary sludge may contain oil, grease, vegetable materials, paper, faecal

materials, sanitary and medical waste, kitchen waste

Treatment process such as activated sludge process, or rotating biological

contactors results in humus sludge or biological sludge (Arnaiz, C., et al, 2006) Humus sludge is the settled product from soluble waste in the primary effluent This is a mixture of microorganism: sloughed bacteria and fungus under living or dead remains Humus sludge has earthly smell and color of dark brown

Humus sludge from biological aerated filters and their variation, which have different types of biological media, share certain characteristics with activated sludge In practice, humus sludge is returned to co-settle with primary sludge in the primary settle

Activated sludge is removed from the activated sludge process Main components

of activated sludge are flocculated and synthesised solids and microorganism (CIWEM, 1995) Due to the rate of recycling and other factors, activated sludge has low TS (1%) with the color ranging from grey, dark brown to black

In the tertiary treatment step, the product is called tertiary sludge It has

fractions in common with secondary sludge, which remains in the effluent of the secondary treatment step and removed in the tertiary step This sludge is

normally transferred to primary tanks and co-settle with primary sludge due to its small amount

Digested sludge, as bio-solids, is the product of biological digestion This process can be performed in the reactor with or without the presence of oxygen,

corresponding in the aerobic or anaerobic digestion processes Bio-solids containnutrient (Jarrell, K.F., et al, 1992) thus they should be considered as a resource They could also contain pathogens, which must be carefully managed because of their impacts on public health Bio-solids are classified due to the level of their contaminant and stabilization As these levels are assessed, the beneficial use of bio-solids will be divided into three sectors: Unrestricted, Restricted, and Not Suitable for Use (Kostenberg, D., et al, 1993)

Combination of different sludge type is commonly utilized in sludge treatment This could be clarified with diverse characteristics and compositions of mixed sludge Regarding AD, the composition of sewage sludge is the mixture of

primary and secondary sludge (Yamada, T., et al, 2005; Noutsopoulos, C., et al, 2012)

2.1.2 Component of sewage sludge

It is important to know characteristics of sewage sludge for its effective

treatment and disposal Generally, sludge includes volatile, organic solids,

nutrients, metals, organic pollutants, and water (Rulkens, W., 2007; Zaha, C., et al,2008) Table 1 summarizes some analyses of sledge components from the

literature

Total solid (TS) content influences the ability of sludge transference in the

sewerage system The higher amount of TS, the more difficultly sludge flows Thus, it is necessary to maintain sludge in liquid stage, which makes sludge flow easier from vessels and through pipes Sewage sludge should be qualified for TS prior to any sludge treatment processes

The value of TS content after being treated can change basing on different

treatment methods After thickening, TS content of sludge will increase up to 9%,and reach 25 – 35% after mechanical dewatering (Milieux, Z.d., 2003)

Table 1 Typical constituents of different types of sludge (Fytili, D., et al, 2008; CIWEM, 1995)

Constituent Unit Type of sludge

Untreated primary sludge

Digested primary sludge

Activated sludgeTotal solids % 2.0 – 8.0 6.0 – 12.0 0.83 –

incineration are used to minimize the VS content (Thanh, N., 2014) AD can biologically convert around 50% of VS to biogas

2.2 Anaerobic sludge digestion

2.2.1 Fundamentals of anaerobic digestion

AD is a process in which organic matter can be biodegraded in the absent of oxygen by a consortium of microorganisms An important product of AD is

biogas, which mainly contain CH4, carbon dioxide (CO2) and traces of other gases (Clemens, J., et al, 2006) AD involves series of biochemical reactions, which can

be divided into four stages, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 3) AD has been used to treat biodegradable organic and produce biogas (5) AD is a sequential process involving several complex

biochemical stages Each stage is consistently performed under activities of interaction of different bacteria In hydrolysis stage, hydrolytic microorganisms

hydrolyse polumer materials to form monomers, such as amino acids and glucose These monomers are then converted to H2, CO2 and short-chain fatty acids such as acetic, propionic acids in the next step, namely acidogenesis In aectogenesis phase, syntrophicacetogenic bacterial metabolize these volatile fatty acids (VFAs) to produce precursors for the methanogenic fermentation In the end, CH4 is formed from either acetate or CO2 and H2 by methanogenic bacterial in methanogenesis step.

Complex particulate organic matter

Soluble organics (simple sugars, alcohol, organic acids, amino acids)

VFAs

H , CO Acetate

Aceticlastic Hydrogenotrophic

Methanogenesis methanogenesis

Figure 1 Anaerobic digestion process of organic maters (Thanh, N 2014)

2.2.2 Current status of anaerobic digestion process

Anaerobic digestion is a natural process which occurs in several environments, such as wetland, rice fields, intestinal tracts of animals, marine or fresh water sediments Humans have applied this process to take benefits as energy, rapid decomposition of organic waste, and stabilized residue for a long time

Over the last two decades, a great deal of the literature has been published on feasible applications of AD for solid waste and wastewater treatment Apart from biogas production, AD brings much greater potential due to more intrinsic

merits, including energy saving, nutrient recovery, reduction of waste organic content to the conventional aerobic digestion (Wilkie, A.C., 2005; Nasir I.M., et al, 2012; Demirel, B., et al, 2005) As a result, extensive application of AD have been only revealed recently with a number of developing designs by focusing on more complicated devices and operational techniques, and increased understandings

of microbiology and biochemistry There are various anaerobic reactor types in practice, of which batch reactors are the simplest configuration The one-stage continuously fed systems, the two-stage and multistage continuously fed systemswere more advanced reactors applied for AD treatment (Ward, A.J., et al, 2008)

CH 4

The evolution of AD applications was also confirmed by a broad range of

potential substrates for this process Anaerobic technology such as single-phase (conventional) and two-phase anaerobic digesters was often used in the

treatment of dairy wastewater for energy product and waste stabilization In term of low content of suspended solids in dairy wastewater, the conventional anaerobic reactors are generally nominated for treatment Currently, variety studies of dairy wastewater treatment have shown a wide range of application of anaerobic reactor designs, such as down-flow film, anaerobic filter, up-flow anaerobic sludge blanket At laboratory scale, the efficient removal of chemical oxygen demand (COD) of these reactors could reach up more than 90%

(Dermirel, B., et al, 2005) The authors also studied the two-phase anaerobic treatment, which was applied for dairy wastewater comprising high

concentrations of non-filtered solids, and lipids The prevalent reactor type was continuously stirred tank reactor (CSTR), and up-flow filter, with CSTR used for acidogenesis stage while up-flow responsible for methanogenesis In comparison between these two processes, the two-phase shows better outcomes with variouskinds of industrial wastewater Sludge from WWTPs has also aroused much consideration since some strict rules of sludge disposals were adopted (Scragg, A.H., 2005) Due to advantages of AD, it has become one of the bright solutions for sludge stabilization and energy production (Arnaiz, C., et al, 2006; Rajagopal, R., et al, 2011; Tomei, M.C., et al, 2009) Current improvements of high-rate

anaerobic system have been drawing more attention on AD performances in agricultural waste treatment, especially animal residue (Ward, A.J., et al, 2008),

which have different features from those of municipal and industrial wastewater (Lorimor, J., et al, 2000) Anaerobic treatment of the poultry and livestock

manure waste, two kinds of agricultural waste, were also of interest due to

increasing concerns of their disposal (Sakar, S., et al, 2009; Demirer, G., et al,

2005), there are more and more investigations of the AD process on them The type of reactors used for livestock manure waste treatment comprise: batch, continuous one stage, and continuous two stage reactors, tubular reactors

2.2.3 Factors influencing the anaerobic digestion process

AD can be sensitive to several operating factors, including pH, temperature, and characteristics of the substrates (figure 2) To optimize the efficiency of AD, thesefactors should be carefully regulated

Figure 2 Factors influencing AD performance

Operating factors Substrate

pH

VFAs

C/N ratio

Alkalinity Temperature

AD processes

2.2.3.1 pH

pH fluctuation can effect biogas yield throughout AD In the early stages such as hydrolysis, acidogenenis, and acetogenesis, pH decrease due to the formation of organic acids Since the methanogenesis phase occurs, pH may increases slightly because of the production of ammonia (Verma, S., 2002) Below pH 6, inhibition

of CH4-forming bacteria may occur, which can disrupt anaerobic process (Castro, H., et al, 2002) The pH inside digesters is an important feature influencing the growth of anaerobic microbes, especially methanogens, through its impact on enzyme activities This is because each group of microorganisms has its own appropriate pH for growth Methanogenis bacteria are seriously sensitive to pH and need a pH range from 6.5 to 7.8 (Sakar, S., et al, 2009) while acid forming bacteria can function in a wider range between 4.0 and 8.5 (Hwang, M.H., et al, 2004) but prefer a pH of 5.5 to 6.5 (Ward, A.J., et al, 2008; Khanal, S.K., 2009) In operation, it is necessary to keep pH close to neutral since methanogenesis is the yeald-limiting step Lime addition is a common technique to overcome pH

reduction

2.2.3.2 Alkalinity

Alkalinity refers to the buffering capacity, which is important for regulating pH in

AD Alkalinity originates from the degradation of organics in the form of CO2, bicarbonate and ammonia (Hwang, M.H., et al, 2004) The equilibrium of CO2 and bicarbonate will resist the dramatic changes in pH Compared to pH, alkalinity or buffering capacity gives more reliability for system stability because the possible

accumulation of VFAs can lead to a reduction in buffering capacity ans pH (Astls, S., et al, 2011).

The pH in an anaerobic system is adjusted by CO2 in gas phase, and bicarbonate

in liquid phase Thereby, pH will decrease if there is a lack of bicarbonate and vice versa In practice, when pH of digester decreases a net strong base, either sodium hydroxide, or calcium hydroxide (Saker, S., et al, 2009) or carbonate salts (Thanh, N., 2014), are utilized They are able to remove CO2 in the gas phase to convert into bicarbonate Bicarbonate can be directly added to reject the lag timeand over organic dosing (Ward, A.J., et al, 2008)

2.2.3.3 Temperature

AD strongly depends on temperature since it affects not only the physicochemicalproperties of substrate in digesters, but also bacteria which is seriously sensitive with any alterations in temperature Thus, it is essential to maintain constant favorable temperatures for the growth of anaerobic microbes (Castro, H., et al, 2002) Water baths or passive solar heating are used for temperature

maintenance; and heat can be added by using heat exchanges in the recycled slurry or heating coils or steam injection in the digester (Ostrem, K., 2004) Any fluctuation of temperature even small change between 30 – 32oC (Ward, A.J., et al,2008), may result in inactivation of bacteria, leading to a decrease in biogas production Furthermore, process failure can be reported at temperature

alteration in excess of 1oC per day (Appels, L., et al, 2008)

There are three temperature ranges investigated for applications: psychrophilic temperature from 10 to 20oC, mesophilic temperature between 20 and 40oC, and thermophilic temperature between 40 and 60oC (Sakar, S., et al, 2009) Since sufficient retention time for CH4-foring bacteria is provided, anaerobic sludge digestion could be operated successfully at psychrophilic temperature as low as

20oC (Ward, A.J., et al, 2008) The main different between mesophilic and

thermophilic digestion is CH4 yield It is studied that CH4 produced by

thermophilic digestion is higher than that by mesophilic digestion in a given digester because of the fact that high temperature is a conducive condition for methanogens growth (Castro, H., et al, 2002; Burton, C.H.T., 2003) Another advantage of thermophilic digestion is to facilitate the balanced fermentation in producing biogas (Del Borghi, A., et al, 1999) The application of high

temperature, however, has some disadvantages, such as the increase of free ammonia or VFAs, which easily results in inhibiting the process (Appels, L., et al, 2008)

2.2.3.4 VFAs

VFAs created during AD are important product and relates to the imbalance of

AD High VFA concentration principally leads to the process failures with respect

to an imbalance among acidogenic, acetogenic, and methanogenic organisms (Boe, K., 2006) Additionally, less effective removal of COD is reported with increased VFA production (Sakar, S., et al, 2009) In the acetogenic stage, the VFA accumulation will result in pH decrease, which adversely impacts on the growth

of methanogens If inhabitations occur in long time, acetogens will predominate

in digesters As mentioned, the addition of buffering is an effective deal since this can resist pH drop and maintain sufficient VFA concentration (Ward, A.J., et al, 2008) While acetic acid is the key substrate for methanogenesis, it if defined thatpropionic and butyric acids are inhibitory to methanogenic bacteria So as to avoid process failure, mornitoring of VFA has been studied to stabilize the overallsystem (Ward, A.J., et al, 2008).

2.2.3.5 Ammonia

The present of ammonia in digester results from breakdown of

nitrogen-containing matter, mainly from protein and urea Ammonia is one of inhibitory substrates to the AD process (Chen, Y., et al, 2008) Between two form of

ammonia, NH4+ and free ammonia NH3 in liquid, free ammonia is defined as the main cause of inhabitation The reason is hydrophobic form of ammonia could easily penetrate through cell walls, causing pH imbalance and enzyme

malfunction (Chen, Y., et al, 2008) This inhibition is generally reported in the methanogenesis stage High concentration of ammonia in digester could affect aciddogenic populations while methanogenic population may lose 56% of its activity (Lettinga, G., et al, 1980) On basis of CH4 production, ammonia has

stronger impact on aceticlastic than hydrogenotrophic methanogens (Thanh, N., 2014) It is suggested that free ammonia concentration should be kept below 80 mg/L, meanwhile ammonium could reach up to 1500 mg/L without making any inhibition (Burtton, C.H.T 2003) pH and temperature are determined as factors affecting the ammonia inhibition capacity through ammonia concentrations

(Chen, Y., et al, 2008) The higher pH is, the more the amount of ammonia is and the less the amount of ammonium is.

2.2.4 Anaerobic digestion in sewage sludge treatment

After wastewater treatment, only liquid can be disposal to the environment whilesolids are collected for further treatment before discharge, of which, sludge is thelargest component in volume (Thanh, N., 2014) Sludge is regarded as both a potential pollutant and a prospective source Accordingly, sludge treatment and disposal should be an integrated part of wastewater treatment Therefore, it is necessary to seek environmentally sound and sustainable methods for sewage sludge handling and disposal Sustainable sludge treatment may be regarded as

“a method that meets requirements of efficient recycling of resources without supply of harmful substances to human or environment” (Fytili, D., et al, 2008) Thereby, sludge management and treatment method should concentrate on threemain aims: (Thanh, N., 2014) complete sludge treatment in term of reducing sludge level, (Fytili, D., et al, 2008) recover valuable products from sludge,

regarding the nutrient used as a soil conditioner or improver, and biogas

production, and (Clemens, J., et al, 2006) the method chosen has reasonable cost.Regarding to disposal of excess sludge and according to the 1990 North Sea Conference, all operation of the sludge dumping at the sea will be banned

Thereby, it is necessary to find replacements for the dumping of sludge at sea The possible solutions are landfill, drying, composting and AD They are towards

to final aim - the transformation of wastewater sludge into the innocuous and easily dewatered form Various studies has confirm that AD is an ideal method

compared to others of sludge destabilization based on its merits as shown in Table 3 This table also shows some downsides of AD, which should also be

seriously considered

Table 2 Advantages and disadvantages of anaerobic sludge digestion (Nasir, I.M.,

et al, 2012; Demirel, B., et al, 2005; CIWEM, 1995; Khanal, S.K., 2009; Jensen, P.D.,

et al, 20115)

Advantages

1) Production of CH4, a renewable resource of energy, recompensing energy for maintaining the temperature of digesting sludge, additionally, heat buildings, drive engines of aerobic blowers or generate electricity that can

be used to run the sewage pumps

2) Net reproduction of mass and volume of sewage sludge during the

conversion of organics into CH4 and CO2 gas and water

3) Transformation of solid content stable, inoculum sludge

4) Beneficial reuse in agriculture – soil conditioner or fertilizers: treated sludge may contain N, P and other nutrients and organics which improve soil quality

Disadvantages

1) High capital costs: large, covered tank, and pumps for feeding and circulating sludge, heat exchanges and compressor for gas mixing

2) The quality characteristics of supernatant from anaerobic sludge

digestion are poor, containing suspended solids, dissolved and

particulate materials, nitrogen and phosphorus