Sludge reduction in a wastewater treatment plant (WWTP) has recently become a key issue for the managing companies, due to the increasing constraints on the disposal alternatives. Therefore, all the solutions proposed with the aim of minimizing sludge production are receiving increasing attention and are tested either at laboratory or full-scale to evaluate their real effectiveness. In the present paper, electro-kinetic disintegration has been applied at full-scale in the recycle loop of the sludge drawn from the secondary settlement tank of a small WWTP for domestic sewage. After the disintegration stage, the treated sludge was returned to the biological reactor. Three different percentages (50, 75 and 100%) of the return sludge flow rate were subjected to disintegration and the effects on the sludge production and the WWTP operation efficiency evaluated. The long-term observations showed that the electro-kinetic disintegration was able to drastically reduce the amount of biological sludge produced by the plant, without affecting its treatment efficiency. The highest reduction was achieved when 100% return sludge flow rate was subjected to the disintegration process. The reduced sludge production gave rise to a considerable net cost saving for the company which manages the plant.

INTRODUCTION

In the biological process of a wastewater treatment plant (WWTP), sludge is being produced as a consequence of the bacterial synthesis at the expenses of a part of the biodegradable substrate (Metcalf & Eddy 2003). Treatment and disposal of biological sludge has been always a considerable item in the overall economic balance of a WWTP management (Zhang et al. 2009). In Italy and in many other countries, in the past years most of the sludge was disposed of in sanitary landfill plants, after proper treatment. In a few cases, it was reused in agriculture or, more rarely, burned in incineration plants.

Recently, environmental regulations have become more stringent in relation to excess sludge quality in view of its disposal or reuse. This has led to a drastic limitation of the available alternatives, and as a consequence a significant increase of the sludge treatment and disposal costs.

Therefore, increasing attention has been given to all the techniques capable of reducing sludge production from activated sludge processes (Déléris et al. 2002; Ødegaard 2004). Sludge minimization can be achieved by either reducing the bacterial synthesis yield or by destroying some of the sludge cells once they have been already produced (Wei et al. 2003; Ramakrishna & Viraraghavan 2005; Pérez-Elvira et al. 2006; Yan et al. 2012; Guo et al. 2013; Wang et al. 2013). In the latter case, several technologies have been tested and proposed, some of which have also been successfully applied at laboratory or full-scale (Sievers et al. 2004).

Many of these technologies are based on sludge cell disintegration. In general, disintegration is the process by which biological cells break down or lose cohesion (Müller et al. 1998; Boehler & Siegrist 2006; Bougrier et al. 2006). This can be done through the application of chemical, physical or thermal processes or of a combination of these forces (e.g., homogenization, ozone oxidation, ultrasonic treatment, thermal hydrolysis, grinding, extruding, centrifugation, high pressure operation). The effect of these processes is the lysis of sludge cells, and the release into the external medium of intracellular compounds which are then solubilized and/or mineralized (Cheng et al. 2012). Recycle of disintegrated sludge to the biological oxidation tank provides an additional substrate (known as autochthonous substrate) to the biomass. Then, the biological process determines oxidation of a part of the overall substrate (both autochthonous and that of the influent) into carbon dioxide, water and ammonia, while the remaining is converted into new cells through bacterial synthesis. The final net result is a lower bacterial cell yield and therefore a reduced sludge production (Chu et al. 2009).

When the disintegration process is applied to the sludge processing line, it is usually located prior to the digestion tank or in the recirculation loop (Liu 2003). Owing to disintegration, sludge cells become more ease biodegraded; thus, the stabilization efficiency of the digestion process is improved and also the biogas production, in the case of anaerobic digestion, is favoured (Salsabil et al. 2010). The disadvantages of these treatments might be the high initial investment cost and also the additional operation and maintenance costs of the disintegration unit.

Although a number of experiences have been reported in the literature on the application of disintegration techniques, further researches are still needed to address the increasing demand from the companies operating the WWTPs to reduce the sludge management, treatment and disposal costs. Furthermore, implementing sludge disintegration technologies in a WWTP usually implies further costs for operating such technologies; therefore, the goal is to achieve a reduction of the overall costs related to sludge management, taking into account both treatment and disposal as well as operating cost items. Since the successful achievement of this goal is strictly dependent on the characteristics of the WWTP, on-site tests at the plant are required, and also in order to assess the best operating parameters of the selected sludge reduction technology (Chiavola et al. 2013).

In the high-voltage disintegration process, the biological sludge flows through a specific pipe system with an internal field of high voltage which produces electric forces. As a consequence, the cell membranes are perforated and finally release intracellular compounds. As compared to the other disintegration methods, this technology requires a low investment cost and a limited energy consumption (35 W per unit), and is easy to be implemented and operated in the plant. The high-voltage disintegration can be installed in several locations in the WWTP; however, most of the experiences carried out so far refer to applications in the sludge processing line, with the aim of increasing biogas production. Only a few cases are reported on its use in the return sludge loop of the water processing line.

The aim of the present study was to evaluate the effectiveness of the high-voltage electro-kinetic disintegration technique applied to the sludge recycle loop from the secondary settlement tank in a small WWTP.

Different percentages of the recycle sludge flow rate were treated by the disintegration process and then returned to the biological reactor. The effects were evaluated in terms of sludge production and net costs (i.e., of the disintegration technique operation and of the final disposal of the treated sludge) sustained by the plant, taking also into account any effect on the performance of the water processing line.

MATERIALS AND METHODS

The WWTP of Borgo Carso

The experimental study was carried out at the WWTP of Borgo Carso (Latina, Central Italy), which is managed by Acqualatina S.p.A. The plant treats a domestic sewage having the following average characteristics: 2,600 population equivalent as actual potentiality, daily average flow rate of 840 m3/d, peak flow rate of 2,520 m3/d, 5-day biochemical oxygen demand of 112 mg/L, chemical oxygen demand (COD) of 200 mg/L and total nitrogen of 18 mg/L.

The lay-out of the plant is shown in Figure 1. As far as the water line is concerned, it consists of the following main stages: pumping, fine screening, denitrification–nitrification–carbon oxidation, secondary settlement, disinfection. The sludge line, which receives the excess sludge wasted from the secondary settling tank, consists of aerobic digestion and drying beds. The plant also receives and treats cesspools.

Figure 1

Lay-out of the WWTP plant.

Figure 1

Lay-out of the WWTP plant.

For the entire duration of the experimental activity, the plant was operated without sludge wasting.

BioCrack technology

The BioCrack technology, supplied by Vogelsang, is based on the high-voltage electro-kinetic disintegration process. In the case of the WWTP of Borgo Carso, it was implemented in the sludge return loop of the plant. Figure 2 shows the main components of the BioCrack and a picture of the system installed at the WWTP of Borgo Carso.

Figure 2

BioCrack components and a picture of the installation at the WWTP of Borgo Carso.

Figure 2

BioCrack components and a picture of the installation at the WWTP of Borgo Carso.

A grinder (named RotaCut) is placed in front of the BioCrack unit for the protection of the internal electrode against stones and long fibres; it also helps to increase the suspension surface area, which will result in a higher efficiency of the electro-kinetic disintegration.

A volumetric pump pushes the sludge to flow into the duct of the BioCrack.

An external centralized control system monitors and regulates operation of the BioCrack with the aim of maintaining constant optimal conditions. Table 1 shows the main technical data of the BioCrack.

Table 1

Main technical data of the BioCrack

Parameter Value 
Power requirement per module 35 W 
Electrical voltage electrode 24 V 
Maximum flow rate 50–80 m3/h for 3.5–5% dry solids 
Maximum pressure 5 bar 
Maximum module length 1.950 mm 
Minimum module width (U-module) 325.5 mm 
Maximum module width (S-module) 492 mm 
Electrical supply voltage 220 V, 50 Hz 
Material housing Stainless steel tube, inside DN 150, PE-coated internal electrode 
Parameter Value 
Power requirement per module 35 W 
Electrical voltage electrode 24 V 
Maximum flow rate 50–80 m3/h for 3.5–5% dry solids 
Maximum pressure 5 bar 
Maximum module length 1.950 mm 
Minimum module width (U-module) 325.5 mm 
Maximum module width (S-module) 492 mm 
Electrical supply voltage 220 V, 50 Hz 
Material housing Stainless steel tube, inside DN 150, PE-coated internal electrode 

Experimental programme

Three different experimental phases were carried out, based on the percentage of sludge flow rate treated by the BioCrack. For instance, the value of the ratio between the sludge flow rate treated by the BioCrack (QB) and the total return sludge flow rate (QR) was changed as follows: (phase 1) QB/QR = 50% (corresponding to QB = 17.5 m3/h); (phase 2) QB/QR = 75% (QB = 24.5 m3/h); (phase 3) QB/QR = 100% (QB = 35.0 m3/h).

The effects of the disintegration process on the efficiency of the water processing line were monitored by periodically measuring the following parameters in the influent and effluent of the plant: total COD, total nitrogen, oxidized nitrogen, and total suspended solids (TSS). Furthermore, the concentration of the mixed liquor suspended solids (MLSS) in the oxidation tank and the value of the sludge volume index (SVI) were determined. Before and after the BioCrack stage, sludge characteristics were evaluated by measuring: total and soluble COD, total and soluble nitrogen, total solids (TS) and TSS.

Sludge production (Px) was determined based on the following equation (Metcalf & Eddy 2003): 
formula
1
where V = biological reactor volume; X = MLSS concentration in the biological reactor; Θc = mean sludge retention time.

For the experimental period, the value of the sludge retention time was calculated based on the net average biomass growth rate measured in the three different phases. However, it must be pointed out that the plant did not operate under steady-state conditions during these phases, because the sludge wasting was stopped; therefore, the sludge retention time does not possess in this case a real meaning and the sludge production calculated for this period can be considered only theoretical.

Analytical methods

All the analyses were performed by following Standard Methods for the Examination of Water and Wastewater (APHA 2005).

Determinations of the SVI were periodically carried out on sludge samples collected from the oxidation tank. These samples were then placed into a graduated cylinder, and the position of the solid–liquid interface registered with time for at least 30 min. Prior to the test, the TSS content of the sample was determined.

RESULTS AND DISCUSSION

As far as the performance of the biological process during the experimental period is concerned, the removal efficiency of total COD and nitrogen remained basically unchanged, as shown by Figure 3(a) and 3(b), respectively. The average values were very similar to those recorded before the BioCrack was operating. Furthermore, the concentrations of COD and nitrogen (in all the forms) as well as of TSS (shown in Figure 4(b)) in the effluent were always far below the limits to be respected according to the law in force and corresponding to the permit given to the plant (indicated by ‘LL’ lines in the figures).

Figure 3

(a) Total COD and (b) nitrogen time-profiles in the water processing line during the experimental study (LL lines indicate the law limits set on the effluent).

Figure 3

(a) Total COD and (b) nitrogen time-profiles in the water processing line during the experimental study (LL lines indicate the law limits set on the effluent).

Figure 4

(a) Settling process of the sludge and (b) SVI and effluent TSS content during the experimental study.

Figure 4

(a) Settling process of the sludge and (b) SVI and effluent TSS content during the experimental study.

Figure 4(a) shows the h(t) profiles measured during the SVI tests, where h refers to the sludge–liquid interface and t is the duration of the test; the secondary y-axis indicates the day of operation when the test was performed. The figure highlights a progressive decrease of the sludge settling velocity with the time of operation of the BioCrack. Furthermore, the SVI slightly increased with time (as reported in Figure 4(b)), particularly at the higher ratio QB/QR. However, the SVI values still remained below 150 mL/g, thus indicating a fairly good sludge settleability.

These patterns registered on the sludge are likely to be attributed to the progressive increase of the mixed liquor concentration in the oxidation tank, due to the absence of sludge waste throughout the experimental activity.

The analyses performed on the sludge collected before and after the BioCrack showed no appreciable differences in the total COD and nitrogen content; by contrast, a decrease of the solid content (TS) was observed in the sludge collected after the BioCrack, with a maximum reduction of about 6% in the third phase, as highlighted by Figure 5. Furthermore, there was a slight difference in the soluble COD of the outlet with respect to that measured in the inlet. These data can be considered as a result of the disintegration process carried out in the BioCrack system which, by causing cell lysis, gave rise to a lower TS content in the sludge after the treatment.

Figure 5

TS in the sludge before and after the BioCrack.

Figure 5

TS in the sludge before and after the BioCrack.

The release of the intracellular compounds in the medium was responsible for the change in the soluble COD content.

All the data reported above seem to indicate that the effects of the BioCrack on the plant operation were apparently negligible. However, clearer information can be obtained through the evaluation of the sludge production during the experimental study, and by comparing this to the value measured prior to the BioCrack operation.

Table 2 shows the MLSS concentration measured in the oxidation tank (X) at the end of each experimental phase. If these values are compared with the MLSS concentration prior to the start of the BioCrack operation, which was about 4,400 mg/L, it is evident that the biomass concentration significantly increased since the beginning of the experimental study. This result can be attributed to the absence of sludge wasting, which determined a progressive accumulation of the solids within the biological reactor. However, the increase was high in the first two phases; then it slowed down in the third phase of the study, corresponding to the highest percentage of sludge flow rate subjected to the disintegration process. Furthermore, a significant decrease of the sludge production (Px) was observed with time. Indeed, the average values measured in each period, and listed in Table 2, are far below the average sludge production calculated for the period when the BioCrack was not operating (equal to about 35 kg/d). A contribution to the observed reduction in the sludge production was also due to the endogenous respiration process, whose rate increased as a consequence of the very long sludge retention time, because the sludge wasting was stopped throughout the experimental study.

Table 2

MLSS concentration and average daily sludge production during the experimental period

Phase  X (g/m3 Px (kg/d) 
1 (QB/QR = 50%) 5,400 10.9 
2 (QB/QR = 75%) 6,700 8.2 
3 (QB/QR = 100%) 6,900 1.1 
Phase  X (g/m3 Px (kg/d) 
1 (QB/QR = 50%) 5,400 10.9 
2 (QB/QR = 75%) 6,700 8.2 
3 (QB/QR = 100%) 6,900 1.1 

The main variation was observed at the higher QB/QR ratios; particularly, a linear correlation was found between the QB/QR value and the sludge production as outlined by Figure 6. This correlation would provide a sludge production of about 22 kg/d for QB = 0 (i.e., in the absence of BioCrack), which can be considered very similar to the average historical data.

Figure 6

Sludge production versus QB/QR.

Figure 6

Sludge production versus QB/QR.

A linear correlation was also found between QB/QR and the average sludge concentration measured in the oxidation tank in the three periods of the study (not here reported); this correlation would lead to the value of 4,400 mg/L for the MLSS concentration expected in the case of QB = 0, which is equal to that measured in this study.

Based on the linear correlation shown in Figure 6, it is also possible to find the optimum QB/QR value at which no sludge production would occur under these operating conditions, which is 109%. However, this value must be considered only theoretical since, as outlined above, the plant did not operate under steady-state conditions during the study.

By assuming a 220 d operation of the WWTP throughout the year, the average annual sludge productions (Mg/y) in the presence and in the absence of the BioCrack were computed. The results obtained are listed in Table 3, where ΔPx indicates the difference between the periods.

Table 3

Annual sludge production in the absence of the BioCrack and during the experimental period

Phase  Px (Mg/y)  ΔPx (Mg/y) 
w/o BioCrack 7.7 – 
1 (QB/QR = 50%) 2.4 5.3 
2 (QB/QR = 75%) 1.8 5.9 
3 (QB/QR = 100%) 0.2 7.5 
Phase  Px (Mg/y)  ΔPx (Mg/y) 
w/o BioCrack 7.7 – 
1 (QB/QR = 50%) 2.4 5.3 
2 (QB/QR = 75%) 1.8 5.9 
3 (QB/QR = 100%) 0.2 7.5 

The significant reduction in the sludge production would give rise to a considerable cost saving for the company managing the plant. Based on the data shown above, this saving depends on the amount of sludge treated by the BioCrack with respect to the total return sludge flow rate, which in turn affects the reduction extent of sludge production.

It must also be pointed out that implementing and operating the BioCrack at the full scale in a WWTP usually requires a very low cost effort. For instance, in the specific case of the WWTP of Borgo Carso, the BioCrack was inserted in the loop of the return sludge, thus exploiting the existing pumps. The increase in the energy consumption was negligible due to the very low power input required by the BioCrack; furthermore, the civil works were very limited. Therefore, the sole cost item for the company was related to the purchase of the BioCrack system. A further cost saving is expected to derive from the reduced operation of the drying system treating the waste sludge.

CONCLUSIONS

The results obtained in the present study demonstrated the capability of the BioCrack technology inserted in the return sludge loop to reduce excess sludge production in the WWTP of Borgo Carso. These results can be expected to be achievable in similar WWTPs.

The significant decrease of sludge production results in reduced costs for sludge treatment and final disposal. As compared to other technologies aimed at reducing sludge production, in the case of the BioCrack the economical budget is more favourable for the managing company due to the lower costs required to implement and run the technology within the WWTP.

The performance of the WWTP was not affected by the recycle of the disintegrated sludge to the oxidation tank.

The BioCrack technology would require a further validation for long-term operation and in the presence of sludge wasting. Furthermore, it would be very interesting to investigate its effects in a WWTP of a larger treatment capacity.

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