Piotr AULICH, Tadeusz MARCINKOWSKI,


Wroclaw University off Technology

Institute of Environment Protection Engineering
 

DISINTEGRATION OF SYNTHETIC SLUDGE

AND EXCESS SLUDGE.

COMPARISON OF RESULTS.

 

 

1.      Introduction

Neutralizing of sewage sludge still remains an open problem. The volume of sewage sludge generated by Polish municipal wastewater plants in the year 2004 amounted to 476,000 tons (dry mass) [13]. It is estimated that untill the year 2010 the annual quantity of sewage sludge generated will exceed 500,000 tons of dry mass [1]. Analyzis of sludge utilizing, performed in surveyed wastewater plants in the year 2003 showed that 36% of sludges was used for ground reclamation, 14% for manufacturing compost and fertilizing preparations, and about 7% in agriculture. Besides, 17% of sludges was directed to landfills and 4% was incinerated; the remaining 22% was utilized in various ways, depending on local needs and capabilities [13]. Many authors claim that processing and eliminating sewage sludge consumes a half, and sometimes even 60% of total costs of wastewater processing [6,7,18]. In order to decrease the amount of sludges, numerous techniques of conditioning, dewatering and preprocessing are being commonly used.

Fig 1. Analysis of sewage sludge utilization in Poland (2003) [13].

2.      Techniques of municipal sewage sludges processing

Taking into account further sludge processing, conditioning techniques can be divided into two kinds:

1.      Techniques supporting processes of mechanical dewatering, that use the following effects:

1.1.effects of polyelectrolytes;

1.2.effects of ultrasounds;

1.3.effects of electric field;

1.4.effects of electromagnetic field (including microwave field);

1.5.effects of magnetic field;

1.6.chemical processing;

1.7.simultaneous effects of polyelectrolytes, together with effects mentioned earlier.

2.      Techniques that support processes of biodegradation, disintegration or that prepare sludge for further processing, that are classified as follows:

2.1.thermal processes;

2.2.biological and biochemical processes:

2.2.1.      enzymatic;

2.2.2.      lysis – latent growth;

2.2.3.      disrupted or sustaining metabolism;

2.2.4.      preying by higher organisms;

2.3.oxidating processes:

2.3.1.      ozonation;

2.3.2.      chlorination;

2.3.3.      advanced processes of oxidation AOT (Advanced Oxygen Techniques);

2.4.acidification;

2.5.alkalinization;

2.6.high-pressure processes;

2.7.disintegration:

2.7.1.      mechanical;

2.7.2.      ultrasonic;

2.8.integrated processes:

2.8.1.      alkalinization or acidification and ultrasonication;

2.8.2.      thermal and acidification or alkalinization.

 

Current researches are directed towards reducing the quantity of sludge generated, as well as increasing cost-effectiveness of processing and reduction of arduousness of thermal and chemical processing techniques.

 

The following solution were tested in technical scale:

1.      mechanical micronizing in high-pressure or ball mixers [14];

2.      ultrasonic processing [2,9,10];

3.      chemical processing utiziling ozone [16], acids or bases [11];

4.      thermal hydrolysis [8,17];

5.      combined thermo-chemical methods, such as Protox, Syntox, Krepro [15];

6.      processes utilizing effects of ultrasonic and electric field, as well as microwave field [4].

 

However, there are some factors that are prohibitive for full scale implementation of such techniques. These are as follows:

1.      high unit consumption of electric energy and relatively low efficiency in case of mechanical micronizing;

2.      problems related to extension of scale, significant consumption of electric energy, low durability and high cost of ultrasonic processing devices;

3.      considerable capital costs related to constructing devices with resistance to highly corrosive environment, as well as generating odours in chemical processing;

4.      significant load of environment, as well as returning considerable part of biological load to processes in thermal hydrolysis;

5.      high capital costs and costs related to utilization of chemical reagents, warming sludges and problematic marketing of products generated by combined thermo-chemical processes.

3.      Synergy in municipal sewage sludge processing

Usually applied solutions most often utilize one of the following effects: mechanical, thermal, chemical or ultrasonic. Integrated effects, such as thermo-chemical, electroacoustic or chemical-electric, are utilized considerably less often.

Data available in literature show that the following phenomena and effects exert a positive influence on sludge processing:

1.      effect of thermal energy;

2.      mechanical effects – mechanical micronization in mills or homogenizers;

3.      effects of electric and magnetic fields;

4.      impact of ultrasonic field, together with acompanying phenomena – cavitation, sonoluminescence, thermal phenomena, etc.;

5.      chemical effects, including the impact of advanced oxygen techniques (AOT);

6.      physicochemical influence of plasm;

7.      other physicochemical effects, such as ultraviolet, microwave and ionizing radiations.

 

Sludge processing device can integrate many actions. It is recommended to utilize the efect of synergy, that is the interaction of individual factors of the process. This effect can be described as follows:

 

                   [A + B + C + D] > [A] + [B] + [C] + [D]          (1)

 

The equation above shows that the total (synergic) effect of components is greater than effects exerted by separate components.

Synergic effects are utilized mainly in medicine, adsorption processes, catalysis and inhibition.

 

The following phenomena are being planned to be utilized as the main factors influencing sludge processing:

          [A] – processes intensifying mass and heat exchange;

          [B] – processes of high-energetic disintegration in plasm;

          [C] – AOT reactions;

          [D] – effects of high-energy sounds and secondary reactions.

 

Equipment for intensifying processes of heat and mass exchange using electric field are well-known and used in chemical industry for quite a long time [12]. Adsorption and desorption of oxygen in fluids can be intensified by 40-50%. In case of rectification, the coefficient of mass transfer can increase by a factor of 2-2,5. Next, in presence of barbotage, maximally threefold increase of the mass transfer coefficient was observed.

Research related to processing bioresistant organic compounds in plasmic channels were carried out by Sugirato et al. [19], Clement et al. [5] and other researchers. Experiments showed that numerous high-energetic phenomena take place in plasmic channels: pyrolysis, photolysis, electro-hydraulic cavitation. The result of these reactions is formation of acitve particles, such as OH•, O•, H•, O3.

Research works related to the effect of ultrasounds were carried out by Bień et al. [2] and Chu et al. [3]. It was found that ultrasonic method of processing consumes considerable amounts of energy, but it has some significant developement potential. There are serious hopes of possibility of implementing ‘soft’ ultrasonic conditioning.

4.      Disintegrator with fluidized bed

In order to carry out a process of integrated effects, such as heat and mass exchange, disintegration in plasm utilizing the impact of AOT techniques, as well as effect of high-energetic sounds, a modified reactor for heat and mass exchange in presence of barbotage was proposed. The device consists of insulated housing, two feeding electrodes, fluidized bed and a system carrying reactive gas.

 


Fig.2. Presumable progression of electrochemical impact on sludge

 

 

Simultaneous action of numerous effects are being utilized: mechanical, chemical, acoustic, electro-hydraulic cavitation, plasm, as well as UV, microwave and electromagnetic radiation.

Under the electric field applied to electrodes, through the fluidized bed, electric current flows as torrential discharges in sludge fluid. Due to these discharges it comes to creation of plasm and electro-hydraulic cavitation, that results in local increase of temperature and pressure. Consequential shock wave causes rapid dispersion and cooling of displaced {extracted} cloud of ionized atoms. As a result of these events, conditions for the synthesis of new compounds are created.

Presumable progression of these phenomena is ilustrated on Figure 2.

5.      Research on electrochemical conditioning

The research was carried out for synthetic sludge and excess sludge from the biological municipal sewage treatment plant.

5.1.    Synthetic sludge

The subject of the research was synthetic sludge of parameters described in Table 1. The following physicochemical parameters of sludge were measured: thickening, CST (capillary suction time), hydratation, ash and volatile substances content. Value of pH, filtration constant and electric energy consumption were also measured. In addition, the quantity of iron transferred from the bed to conditioned sludge was determined.

The research involved raw sludge, sludge circulated for 80 minutes and sludge conditioned electrochemically for 10 minutes (including circulation) in presence of barbotage with air. Besides, for conditioned sludge its content of dry mass, ash, volatile substances and iron was determined after 6 days of seasoning.

 

Table 1. Composition of synthetic sludge.

Parameter

value

unit

dry mass

ca. 2

%

proteins

8,2

% d.m.

digestible carbohydrates

81,8

% d.m.

     – incl. sugars

5,7

% d.m.

oils

0,5

% d.m.

fats

0,7

% d.m.

nutritional cellulose

4,2

% d.m.

 

5.1.1.         Results of synthetic sludge examination

Results of synthetic sludge examination are summarized in Table 2.

Examined sludge contained initially 2.02% of dry mas. After circulation this content increased to 2.11%. After conditioning the same sludge contained already 2.14% d.m.. Then, after six-days seasoning condtioned sludge showed the content of dry mass at the level of 2.03%.

The content of volatile substances in final sludge was 94.4% (dry mass). After circulation it decreased to 91.1% d.m.. Conditioning carried out for ten minutes caused decrease of volatile substances content to 89.3% d.m.. Repeated measurement after 6 days of curing showed the result of 88.5% d.m..

Ash content in final sludge was 7.6% d.m.. After circulation it increased to 8.9% d.m.. Ash content in conditioned sludge increased to 10.7% d.m., and after six days of seasoning it reached the value of 11.5% d.m..

Iron content of sludge was also analyzed. In reference sludge it amounted to 418.6 mg/kg of ash. After circulation process the amount of iron increased to 11.2 g/kg of ash. In conditioned sludge the content of iron in ash amounted to 228.6 g/kg of ash, and after seasoning it increased to 273.6 g/kg of ash.

pH value of initial sludge were 5.51. In circulated sludge it decreased to 5.12, and conditioned sludge showed the value of 6.46.

Due to technical reasons, the specific resistance was not determined and research was limited to measurement of filtration constant. For particular conditions a filtration constant is directly proportional to specific resistance of filtration. Along with increase of filtration constant, the specific resistance of filtration also increases. For initial sludge the filtration constant amounted to 3.22. In case of circulated sludge, it increased to 15.1 (4.72-fold increase). For conditioned sludge it reached the value of 1.44 (45% of filtration constant for reference sludge).

CST parameter (capillary suction time) for initial sludge amounted to 523 s. For circulated sludge it increased to 652 s (an increase of 25%), and for conditioned sludge decreased to 419 s (decrease of 20% in relation to reference sludge).

Thickening ability of synthetic sludge subjected to examination was very poor and only for conditioned sludge it  was possible to achieve a thickening of 980 cm3/dm3, that maintain for 2 hours. After 24 hours this density reached a level of 950 cm3/dm3.

 

 

Table 2. Summary of results of research on synthetic sludge.

Parameters

Units

Initial

Circulated for

80 min.

Conditioned for 10 min.

Conditioned after seasoning

Dry mass

%

2,02

2,11

2,14

2,03

Volatile substances

% d.m.

92,4

91,1

89,3

88,5

Ash

% d.m.

7,6

8,9

10,7

11,5

Iron in ash

g/kg

0,42

11,3

228,6

273,6

Reaction

pH

5,51

5,12

6,46

-

Filtration constant

s / m3

3,22

15,1

1,44

-

CST

s

523

652

419

-

Consistence after 24 h

cm3 / dm3

1000

1000

950

-

Electric energy consumption

kWh / dm3

-

-

0,005

-

kWh / kg d.m.

-

-

0,25

-

Exhaustion of bed material

g / kWh

-

-

128

-

 


Fig 3. Comparison of selected parameters of synthetic sludge.

5.2.    Excess sludge from the municipal sewage treatment plant

The subject of research was circulated excess sludge from sewage treatment plant utilizing the Biolak von Nordenskjöld technology. The following physicochemical parameters of sludge were measured: thickening, capillary suction time (CST), pH, dry mass content, ash, volatile substances and COD. Determination of COD was performed in 24 and 48 hours after the process of conditioning. Additional parameters that were measured included a specific resistance of filtration, rate of filtration and its efficiency.

Sludge was conditioned for 5, 20 and 60 minutes in presence of barbotage with air.

5.2.1.         Results of excess sludge examination

After 120 minutes of thickening process, reference sludge reached a consistency of 879 cm3/dm3. Conditioned sludge reached consistency of 941, 939 and 990 cm3/dm3, for times of conditioning equal to 5, 20 and 60 minutes, respectively.

CST for the sample of reference amounted to 7 s. For samples of conditioned sludge it increased to 15, 27 and 46 seconds, for the times of conditioning 5, 20 and 60 minutes, respectively.

The reaction value for the reference sample was 6.39 pH. For samples of conditioned sludges it increased to pH = 7.8, pH = 7.82 pH and pH = 8.22 for periods of conditioning: 5, 20 i 60 minutes, respectively.

 

Table 3. Summary of results for excess sludge.

Parameters

Units

Initial

Conditioned for

5 minutes

Conditioned for

20 minutes

Conditioned for

60 minutes

Consistency after 120 min.

cm3 / dm3

879

941

939

990

CST

S

7

15

27

46

Reaction

pH

6,39

7,38

7,82

8,22

Dry mass

%

1,06

1,0

1,0

1,14

Ash

% d.m.

38

42

48

60

Volatile substances

% d.m.

62

58

52

40

COD after 24 h

mg O2 / dm3

48

120

131

271

COD after 48 h

mg O2 / dm3

61

180

340

516

Specific resistance of filtration

× 1010 m / kg

0,9

10,6

20,9

17,0

Average rate of filtration

cm3 / s

1,12

0,14

0,041

0,040

Efficiency of filtration

kg / m2h

29,98

5,41

2,22

2,21

 

Dry mass content for reference sludge was 1.06%. For sludge conditioned for 5 and 20 minutes it decreased to 1.0%, and for sludge conditioned for 60 minutes increased to 1.14%..

Ash content for the reference sample amounted to 38% d.m.. After processing it reached values of 42, 48 and 60% d.m., for times of conditioning 5, 20 and 60 minutes, respectively.


 


Fig 4. Comparison of selected parameters of excess sludge

 

Volatile substance content in reference sample was 62%. After conditioning it decreased to 58, 52 and 40% for times of conditioning 5, 20 and 60 minutes, respectively. 

COD was determined on 24 and 48 hours after the conditioning process. In case of reference sample, the value of COD amounted to 48 and 61 mg O2/dm3 for 24 and 48 hours, respectively. For the sample taken after 24 hours COD increased to 120, 131 and 271 mg O2/dm3, for times of conditioning 5, 20 and 60 minutes, respectively. For the sample taken after 48 hours it increased to 180, 340 and 516 mg O2/dm3, for times of conditioning 5, 20 and 60 minutes, respectively.

Specific resistance of filtration for reference sludge was 0.9 x 1010 m/kg. It increased to the values of 10.6, 20.9 and 17 x 1010 m/kg for times of conditioning 5, 20 and 60 minutes, respectively.

Average rate of reference sludge filtration, for the measurement range defined as 30 – 90 cm3, for 100 cm3 of sludge and measurement gradation set as 10 cm3, turn out to be 1.122 cm3/s.  It decreased to 0.140, 0.041 and 0.040 cm3/s for times of conditioning 5, 20 and 60 minutes, respectively.

Efficiency of reference sludge filtration was 29.98 kg/m2h. It decreased to 5.41, 2.22 and 2.21 kg/m2h for times of conditioning, respectively, 5, 20 and 60 minutes.

6.      Conclusions

1.      As a result of disintegration of floccules structure, sludge dewaters harder, however destroyed structure of flocks enables easier stabilization, therefore the electrochemical conditiong process has positive effect on sludge stabilization, eg. in aerobic or anaerobic processes.

2.      In case of excess sludge, conditioning had a negative effect on such processes as: sludge thickening, CST, resistance of filtration, average rate of filtration and its effectiveness. Values of these parameters increased along with prolonging time of processing. Dry mass content changed only slightly.

3.      The longer the process of conditioning, the higher was the increase of ash content and the lower content of volatile substances in dry mass – sludge was being mineralized.

4.      A part of organic substances was transferred to supernatant, therefore increasing the value of COD – the longer the time of conditioning process, the more was the increase.

 

 

References

 

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