Anaerobic waste collected from toilets and contains high

Anaerobic
treatment systems for treating faecal sludge

Amandeep Singh, M.sc WATENV

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Abstract:

Faecal sludge is waste collected from toilets and
contains high organic contents compared to general domestic wastewater, hence
treatment of the faecal sludge as wastewater is not economical and efficient.Anaerobic
treatment of faecal sludge is efficient as there is enormous generation of  methane (CH4) during the digestion
process which can  be converted to
biogas, as a source of energy.

Introduction:

Faecal Sludge
and its characteristics:

 

Faecal  sludge 
existing in a raw  state or  in some  digested 
state  is a solid , semisolid  and  is
formed from  the collection, storage or  combination of  human excreta 
and blackwater, which may include household, industrial or other forms
of  drainage water in it . Faecal sludge  is vastly different to itself  in  terms of 
texture, concentration and quantity. It contains number  of matters  including  solids and  nutrients which are col­lected from  septic tanks, pit latrines and low flush unsew­ered
public toilets. Faecal sludge defecated without any treatment spreads pathogenic bacteria’s into onsite and open places that
 may result  to health  hazards to living organisms and human beings. (Linda
Strande 2014)

Anaerobic
treatment for Faecal Sludge:

 

Anaerobic
treatment of faecal sludge is the most apt method employed in decentralized
sanitation systems especially when the objective is resource conservation and reuse.
Anaerobic digestion is suitable for faecal sludge treatment in all
environmental conditions even when faecal sludge is highly concentrated. The anaerobic
effluent at a later stage is usually needed to be managed and handled further for
reuse, discharge and safe disposal (Kujawa-Roeleveld, Zeeman 2006).

 Lack of oxygen
describes anaerobic conditions. Anaerobic degradation takes place in FS
Treatment Plants in which the systems avail to the depleted level of oxygen . Common
applied anaerobic systems are septic tanks, settling tanks and waste
stabilization ponds. Beside being an useful method to stabilize faecal sludge,
the anaerobic digesters

produce less sludge (i.e. microbial biomass) in the
anaerobic mechanism. Anaerobic digestion undergoes a wide process consisting
primarily of hydrolysis, acidogenesis, acetogenesis and methanogenesis. Hydrolysis
is an chemo-enzymatic action in which complex organic compounds are broken down
into simpler compounds. In addition to that , biomolecules like proteins,
polysaccharides and lipids are converted into amino acids, monosaccharides and
fatty acids. In acidogenesis , methogenic substrates like H2, CO2,
methanol, methylamines, and acetate are composed from amino acids,fatty acids
and sugars with the help of acidogenic microorganisms. In the course of methanogenesis,
acetate breaks down to methane and carbon dioxide and methane is formed by the
use of hydrogen and carbon dioxide. The stimulation occurence of methanogenesis
is ideal at thermophilic (49-57°C) and at mesophilic (30- 38°C) temperature.  The following equations will show methogenic
equations (Linda Strande 2014)

Equation 1: 4H2+ CO2 g CH + 2H2O

Equation 2: CH3OH + H2 g CH4
+ H2O

Equation 3: CH3COO- + H2O g CH4
+ HCO3 (Madigan et al., 2003)

Process in Anaerobic treatment:

 The growth rate
of methanogens which utilize the hydrogens needs to be kept optimal for
consistent operations .Whenever the mechanism slows down, then the unstable
fatty acids created by acidogens will be formed up in the inner surface of  reactor causing  a low pH that disrupts methanogenic activity.
Monitoring the pH has to be ensured at the first for conformity and optimal
performance.

Primary sludge and waste generated sludges are
digested in decentralized treatment systems like upflow anaerobic sludge
blanket reactors (UASB), plug flow reactors (PFR) and continuously stirred
reactors (CSTRs). Industrial effluents and vastly laden sewage effluents   specifically opt anaerobic treatment plants (Arthur
et al., 2011).

 Settling-thickening
tanks may be preferred in such a metabolism. The settling of faecal matter
,retention time and temperature determine the limits for the anaerobic
mechanism. Then the organic matter is broken down and releases gases. Gas
bubbles are observed in freshly taken feacal content because it was not decayed
properly and contains traces of wastewater in it.In such a case settling-thickening
tanks should be adopted (Heinss et al., 1998; Vonwiller, 2007).

Figure 1: The flow chart below depicts the process involved in
the anaerobic treatment of waste.

 

Factors considered for the design of anaerobic
treatment systems:

Anaerobic digesters major design factors are the
loading pattern, temperature and hydraulic retention time. Well functioning
conditions need to established in order to achieve the best design for the most
preferred execution of anaerobic digesters, Among these conditions are solids
retention time, pH, alkalinity, temperature, HRT, toxic substances and the presence
and existence of trace elements and nutrients.

While selecting an quintessential reactor, it is vital
to predetermine the concentration of  organic matter that one can assume easily, to
permit sufficient and lengthy HRT for the decompositions to take place. For
instance in systems where SRT would equal to the the HRT. . Rise or reductions in
HRT causes an upsurge or downturn in the proportion of hydrolysis,
acidification, methanogenesis and acetogenesis (Metcalf and Eddy, 2003). Hence
it is necessary to be vigilant to maintain HRT to avoid reactor failures.
Factors like temperature plays a great role in methane formation. On the other
hand, temperature alters the chemical and physical aspects in reactors as in
gas transfers, solubility of salts and inactivation of harmful pathogens (Linda
Strande).

Difference between aerobic & anaerobic systems:

 

Anaerobic bacteria transform organic matter in the
faecal sludge into biogas that contains large amounts of methane gas and carbon
dioxide whereas aerobic processes use the bacteria then break down the sludge
into organic compounds. Biogas is composed of methane (55- 75%) and carbon
dioxide (30-45%) (Arthur et al. 2011).  Anaerobic
treatment does not use air to decompose the organic matter whereas aerobic
treatment does. Anaerobic treatment has enormous potential to produce energy
with the help of Biogas (methane) that can be used as an ideal fuel. Collecting
methane is better rather than to release methane as methane is a greenhouse gas
and has a bad odor.

Advantages of biogas accumulation by anaerobic
systems:

 

Proper and efficiently biogas producing systems manage
to return good outcomes and a wide variety of assistance for the community,
society and the environment in common. For example generation of useful energy
like light ,elec­tricity and heat. Among other benefits are conversion of organic
matter into a superior fertilizer which is good in quality. Hygienic
surrounding are upgraded by declining the number of pathogens, flies and worm
eggs especially for  those women who are employed
in cooking and have to gather firewood from forests in rural areas. Conservation
and protection of ecology by saving the water, soil, air, and the natural vegetation.
Small scale profits can be targeted by fertilizer replacements and change in
the usual source of energy with supplementary modes of income and raised outcomes
from agro-based products and animal livestock . Profits from large scale
outcomes by large energy houses, ecological shelter and im­port replacements (Afifah,
Priadi 2017)

 

Co-treatment of
faecal sludge

 

For volume
deduction, sludge stabilization, and enhanced dehydration to water, co
treatment is suitable. Some favourable options for it are upflow anaerobic
sludge blanket reactors (UASB), stabilization ponds and anaerobic digesters Through
the production of biogas anaerobic treatment can lower expenditures avoiding
excessive financial burden and being an good alternative for production of
electricity and heating. Thermophilic digestion is an superb method to
eliminate pathogens (Metcalf & Eddy 2003). The
aspects of feacal matter is to be safely and precisely taken into consideration
 as less settled Faecal matter aim to
have greater amounts of biodigestable organic content. There are presently some
defined uses and applications that utilize biogas as valuable source of energy.
Hence lots of research for the development 
of anaerobic systems is being done 
to aim for high strength FS (Strauss et al., 2006). Co-treatment
is suitable for sludge collected from septic tanks and other digested sludges.
But lower the amount of biodigestable organic matter in digested FS may result to
little biogas generation but greater collection of solids forming an important
operational   investment with scarce
number of outcomes (Still and Foxon, 2012). Feeding is the most vital step in
anaerobic treatment. It ought to be supplied gradually and slowly to prevent any
sudden shock and enormous disruptive overload (Heinss and Strauss, 1999;
Metcalf and Eddy, 2003; van Lier, 2008). The highest value of OLR for the design
should not  exceed to bypass the limited loading  of the process specifically to great strength
.Thus overloading can be avoided by handling faecal matter .In a research
study, it has been observed that 0.25%  of high strength FS that’s roughly close to 10
tankers of  volume 5 m3 per day carried carried
 organic matter identical close to
139,000 p.e. which clearly resulted overloading of a UASB plant of 100,000 p.e
capacity .For anaerobic co-treatment , anaerobic digesters are suitable.
Permissible loading rates for mesophilic digesters (operated at 35?C) depend on
the operational conditions but can reach. A value as high as 1.6-2.0 kgVSS/m3/d
can be avoided by a permissible loading intensity at a temperature of 35?C
(Heinss and Strauss, 1999; Metcalf and Eddy, 2003). Maximum feeding rate of
design should be restricted that is reliant on the associated SRT. Faster hydrolysis
rates are achieved by using Thermophilic anaerobic digesters (49-52?C) giving
greater outcomes in higher biogas generation (Angelidaki et al., 1993). But
it is affected by low temperatures and financial expenditure on working and
maintenance are quite more as compared to mesophilic digesters which becomes
not a preferable option for low-income nations (Heinss and Strauss, 1999). If
adopting low loading systems of 0.6 kg BOD5/m3/d can be adopted, one is likely
to use cost effective technologies for FS like waste stabilization ponds. But implementation
of such a system requires investment and excecution costs that has to be
pre-evaluated that can be expensive because 
they require abundant land requirements and huge functioning costs
because of regular desludging is required. Although methane can lost to the
ecology if not properly handled (Linda Strande 2014)

Resource Recovery:

 

Research
work developed in this area includes the potential of anaerobic treatment,
which is to get resource recovered from the treatment of faecal sludge and to
utilize a reliable and environment friendly natural resource. Innovative
methods have been developed for resource recovery, and progress is done to use
dried sludge as a form of fuel.

Presently
2.7 billion people globally need effective solutions in each field of
management and planning to provide onsite sanitation technologies so that billions
of people are served in the future. Being honest and responsible will help all
developing countries to provide smart,innovative and excellent results by
learning from previous problems and also searching for future possibilities from
the failures in the past (Linda Strande 2014)

Conclusion:

In this
era, there is a high need for clean, reliable and environmental friendly
treatment technologies due to the increasing stress on energy production. The
anaerobic treatment resolves problems of the developing countries, considering
the efficiency, costs, space required and disposal. In addition to that, we can
preserve and create energy from biogas which is an end product of faecal
sludge. Anaerobic treatment should be widely adopted to eliminate the complex
methods used in aerobic treatments to treat faecal sludge.