– Volume 20 ( 2014 ) No . 2 7 APPLICATION OF THE FLOCCULATION PROCESS FOR THE MUNICIPAL SEWAGE SLUDGE TREATMENT

All municipal sewage treatments using classic treatment chain create an amount of organic sludge. The stabilization/neutralization of them is essential before any conditioning or reuse. For agronomical recycling or energy-giving optimization, the sludge needs to correspond to some standards. That's why, it exist several treatment of stabilization, packaging and water extraction of sludge. This study deals with sludge chemical conditioning by flocculation process. In the first hand, the flocculation process allows to break up colloidal aversion between particles in sludge. This process is possible by charge neutralization thanks to synthetic or natural organic flocculation which is often cationic since the ionic force of sludge particle usually is negative and the flocculants can be used with lowest dose than common coagulant like aluminum sulfate or ferric chloride. Thanks to laboratory experiments by analysis of several doses of synthesis cationic flocculants, it has been possible to get performance on total suspended solids (TSS), by spectrophotometry measurement and consistency of sludge cake. Graphical analyses allow making comparison between some organic cationic flocculants and select the best one for this type of sludge and the kind of treatment used. The aim is to improve quality of sludge, quantity of filtrated volume, and quality of water.


INTRODUCTION
In the Romanian strategies for prevention and reduction of environmental pollution are included the management of sewage sludge (part of the European strategy).In this strategy are include clear references to a number of quality indicators that must fulfill sludge leaving the wastewater treatment plants.Thus, the reduction of sludge moisture (not more than 65 %) is very important in assessing of sludge quality for disposal and/or utilization [1].Also, over 40 % of Romanian population lives in rural areas where sewage infrastructures is missing or under implementation.In this sense the development of sewerage facilities are essential to reduce the environmental pollution (by disposal reduction of untreated wastewater in natural emissaries), improving living conditions and protect the human health [2,3].
Laying out "a code of good practice on sludge capitalization" will represent a starting point in reviewing quality standards and can improve methods for sludge management [4].
Coagulation/flocculation process allows destabilizing the colloidal materials and causing the small particles to agglomerate into larger flocs.Most of coagulant can be used combined with other flocculants product [5][6][7][8].
Use of physico-chemical product alone or together is not exactly described in a model whereas it exist several current studies on it since the choice of the coagulant or flocculants for the treatment can be realized only by laboratory experiments [8][9][10][11].
In this study, we have analyzed different kinds of flocculants.The aim is to evaluate the effectiveness of flocculation process using cationic organic flocculants for the treatment of municipal sewage sludge.The optimum conditions such as flocculants dosage were investigated by laboratory experiments thanks to series of sewage filtration.The result of filtrated volume, total suspended solids (TSS) or consistency of sludge cake (which is evaluated by analyzing moisture) permits to compare and choose the best flocculants [1].

Municipal sewage sludge sampling
In Romania, after wastewater treatment, result large quantities of sludge's (more than 96400 tons as dry matter) [2].In the case of municipal wastewater treatment plant from which the sludge's samples have been taken can be specify that approximatively 200 m 3 are daily treated (by using flocculation process).
One sample of sewage sludge comes from the thickening process of primary and secondary excess sludge before anaerobic treatment and the other after anaerobic treatment, before dewatering system.Samples of raw sludge present respectively 2.16 % and 3.96 % dry solid matter.
The first part of the experimental work consisted of preparing the flocculants solutions and sludge samples (Figure 1).The method to analyze the effect of flocculation on sewage sludge after conditioning is gravitational filtering method.The gravitational filter used during laboratory determinations is presented in Figure 2. To characterize the sludge before or after flocculation and filtration, materials used are moisture balance (Mettler Toledo's HB43, Figure 3) to analyze the percentage of solid dry matter in sludge before conditioning and spectrophotometer (LANGE DR 3900) to determine the TSS concentration.The total suspended material determination was according to the standard EN 872 CSN, 2005 [4].
More, the compression filter with plastic funnels permits to recovery filtrated volume of water after filtration and gets back sludge cake.
The different cationic organic flocculants were in liquid form.The concentrations of these flocculants were 0.05 %.

Fig. 3. Moisture balance and spectrophotometer (laboratory materials).
In the literature the organic coagulants and flocculants products are easier use with weak dose than inorganic salts, commonly used for coagulation and flocculation.Even if, it is more expensive, this doesn't modify the characteristic of sludge such as pH or alkalinity, which were not studied here [6].

The used flocculants
The flocculants uses in this experiment are synthesis cationic polyacrylamide.There are 7 kinds available, respectively: Ecofloc 90481FS, Ecofloc 90491FS, Ecofloc 80002LX1, Ecofloc 80802LX1, Ecofloc 60202LX1, Ecofloc 80202LX1 and Ecofloc 60002LX1.These different types of synthesis cationic polyacrylamide depend on charge type, density and molecular weight.These general chemical descriptions are copolymers of acrylamide with quaternary cationic monomers.The cationic polyacrylamide has variable molecular weight and high cationic charge density (available positive charged sites).The main use of this polyelectrolyte is on a wide variety of wastewater and wastewater sludge.This flocculants can be used for a wide range of applications as mechanical dewatering and thickening.They are operative for a wide range of pH 4-9.
Generally, the charge of particles sludge is anionic so use of cationic flocculants permits to ionize particles to form positive sites improving agglomeration [6].

Experimental sludge conditioning
Approximately 100 mL of (with 2.16 % and respectively 3.96 % dry solid matter for the two types of analyzed sludge) sludge sample are put in different recipients.Several doses of flocculants are added in each container.
Once the flocculant was put in sludge, it was mixed at 200 rpm for 15 min.After that the mixture was gravitationally filtered 5 min.From now on, it is possible to get back filtrated volume of water and evaluate the sludge cake.

RESULTS AND DISCUSSION
The conditioning of sludge manages to improve the dewatering systems.For this reason, it is important to get an important volume of water after filtration.
The analyze is based on two important parameter:  More filtrated volume is important, better is the consistency (reduction of the humidity) and the quantity of sludge;  Less the concentration of suspended matter is important, better is the quality of sludge.
In the experimental work were used two doses for each of the seven flocculants use 35 mL (results presented in Table 1) and 30 mL (results presented in Table 2).Flocculants solution concentration (in all seven cases) was 0.05 ppm.
The sludge cakes obtained after filtration process was analyzed by consistency, without humidity determination (%).The sludge cakes were analyzed introduced in three quality categories classification: good, medium and low (Tables 1 and 2).According to the correlation between filtrated volume/TSS, it seems that Ecofloc 80202LX1 and Ecofloc 60002LX1 are the best flocculants for this sludge.In order to select one of these flocculants, the dose must be studied.

Filtrated volume
Regardless of the injected dose, Ecofloc 80202LX1 and Ecofloc 60002LX1 have the higher relation filtrated volume/TSS.
But with these selected flocculant, it is important to look at the injected dose.With the first dose at 35 mL, filtrated volume of Ecofloc 80202LX1 and Ecofloc 60002LX1 flocculant is respectively at 64 and 65 mL.
With the second dose at 30 mL, filtrated volume of Ecofloc 80202LX1 and Ecofloc 60002LX1 flocculant is respectively at 59 and 54 mL.Consequently, for this dose filtrated volume is smaller than with the first dose.In order to choose the best dose, it is important to analyze the charge of TSS.

Total suspended solids (TSS)
The Romanian Decree of 22th December 1994 describes the regulation on wastewater sludge characteristics.For the suspended matter in sludge after treatment, the regulation must be inferior at 30 mg/L that is equivalent to purification rate of 90 % (Figure 4).In this way, Ecofloc 80202LX1 and Ecofloc 60002LX1 remains best flocculant.With the first dose, TSS is higher than with the second dose (Figure 5).
For Ecofloc 60002LX1, TSS is the same for both, but the filtrated volume is the smallest with the second dose.The best reagent presents the smallest dose to form strongest flocs.
From the Figure 4 can be observe that the Ecofloc 60002LX1 flocculant is the best for the treatment of this sludge but is necessary to look at the consistency of this sludge.These aspects result from analyzes of the filtrated volume 65 mL but also from the low concentration of TSS in the filtrate (20 mg/L).The graph from Figure 5 reveals that Ecofloc 60002LX1 and of Ecofloc 80202LX1 were obtained the best results in the case of filtrate volume and TSS concentration.Also from the both graphs can be observed that these two flocculants, at both doses (35 and 30 mL), were obtained the best results.The graph from the Figure 6 present the comparison from the results obtained in the case of Ecofloc 80202LX1 and Ecofloc 60002LX1 utilization.From this graph analyses can be concluded that Ecofloc 80202LX1 is better that Ecofloc 60002LX1 in special because it was obtained a difference of 5 mL for filtrate volume and 1 mg/L at a lower dose (30 mL) which can means less cost in industrial sludge treating.

Sludge cake
The sludge cake analyzes is also an important aspect during flocculation process efficiency evaluation.In this study the sludge consistency, after treatment, were analyzed by a simple touching of the sludge and introduced in three quality categories classification: good, medium and low (Table 1 and 2).In the future research will be taken into consideration the humidity determination for each sludge cake, after treatment with flocculants [4].
For Ecofloc 80202LX1 and Ecofloc 60002LX1, the consistency of sludge cake seems good for the first dose.
With the second dose, the Ecofloc 60002LX1 flocculant gives form of less good sludge than the Ecofloc 80202LX1 flocculant.The color of the sludge cake doesn't differ a lot between the both of sludge cake select (Figure 7).Fig. 7. Sludge cake after the experimental treatment.

CONCLUSIONS
The aim of this study allows showing that it is possible to improve the dewatering potential of sludge improving the quality of flocs by applying the flocculation process.The best reagent is the one, which has the smallest dose, can give a form to strongest flocs in order to obtain a good consistency.
In the laboratory evaluation of the flocculation process, for a specific sludge, is very important to use more flocculants at different dose.These aspects can permit a better evaluation of the flocculants applicability for different types of sludge's.
In the case of two types of flocculants use (Ecofloc 80202LX1 and Ecofloc 60002LX1) were obtained the best result regarding to the quantity and quality of the filtrate.
Using two doses of flocculants and obtaining good results in case of lower dose (30 mL) can have positive effects on the costs of implementing the flocculation process at industrial level.
The relation between the filtrated volume and the TSS permits to show, following the used of 7 different flocculants.
In the future researches will take into consideration the using of more concentrations for the analyzed flocculants category and a better evaluation of more quality indicators for the treated sludge's [4].

Fig. 4 .
Fig. 4. Results obtained after flocculation treatment applied at 35 mL dose for each flocculants.

Fig. 5 .
Fig. 5. Results obtained after flocculation treatment applied at 30 mL dose for each flocculants.

Table 1 .
Experimental results obtained at first dose (35 mL).

Table 2 .
Experimental results obtained at the second dose (30 mL).Thanks to the relation filtrated volume and TSS, it was possible to choose the best flocculant.For filtrated volume determined, a low concentration of TSS causes high relation.As a result, it was decided to give advantage of high filtrated volume and low concentration of TSS.