Case Studies For Potato Processing Factory

Case Study I. This study examines the first EGSB operating in a German potato processing factory [13]. A wastewater flow of 1700 m3/day passed through a screen and a fat separator into a 3518 m3 balancing tank (weekly balance 30% constant retention) that also served as an acidification tank. Owing to the high retention time, it may be assumed that a nearly complete acidification took place, between 40 and 50% related to filtered COD. The methane reactor had a height of 14 m with a water volume of 750 m3. The feeding of the reactor occurred at a constant rate from a conditioning tank (pump storage reservoir), where the recirculation flow mixed with the influent and the pH was adjusted to 6.6, using sodium hydroxide. The effluent from the methane reactor passed through a lamella separator for the removal of solids, which could also be placed between the acidification and methane reactor. The anaerobically treated wastewater was fed into the municipal wastewater treatment plant.

With an average filtered COD of 3500 mg/L in the influent, the efficiency of the anaerobic treatment was 70-85%, resulting in a biogas production with about 80% methane content. The concentration of filterable solids in the influent fluctuated between 500 and 2500 mg/L. According to operational experience in this anaerobic system, these values have not caused any considerable deterioration of the pellet sludge structure during operation.

Case Study II. This study addresses the anaerobic treatment of wastewater from the potato processing industry. A Biothane UASB reactor and Biobed EGSB reactor were installed at two different potato processing facilities in the Netherlands [14]. The first example is Smiths Food, which produces potato chips. They chose the Biothane UASB anaerobic treatment process for bulk COD removal from their wastewater and aerobic final treatment to meet the discharge limits. Figure 17 shows the flow scheme of this process. Coarse solids are removed in a parabolic screen (mesh size 1 mm). After this screen, the water enters a preclarifier designed at a surface load of 1 m/hour for removal of suspended solids and residual fat, oil, and grease. The settled solids are dewatered in a decanter and the water flows by gravity into a buffer tank of 400 m3. From the buffer tank, the water is pumped to a conditioning tank for pH and temperature correction. Conversion of COD takes place in the Biothane UASB reactor. The total anaerobic plant has a COD removal efficiency of approximately 80%. The remaining COD and kjeldahl nitrogen is removed in the aerobic post-treatment.

The final COD concentration is less than 100 mg/L and the Kj-N concentration is less than 10 m/L. The final effluent is discharged to the municipal sewer. The performance of the combined UASB anaerobic-carrousel aerobic wastewater treatment plant of Smiths Food is specified in Table 11.

The second example is Peka Kroef, which produces potato and vegetable-based half products for the salad industry in Europe. Owing to the specific characteristics of the resulting wastewater (low temperature, COD load fluctuations, COD composition fluctuations, high suspended solids concentration) an alternative for the conventional UASB, the EGSB technology, was tested. Extensive laboratory research showed good results with this type of anaerobic treatment at temperatures of 20-25°C.

Figure 18 shows the flow scheme of the EGSB process at Peka Kroef. The wastewaters from the potato and the vegetable processing plants follow similar but separate treatment lines. Coarse solids are removed in parabolic screens and most of the suspended solids in a preclarifier. The settled solids are dewatered in a decanter and the overflow is fed into a buffer tank of 1000 m3. The anaerobic plant consists of two identical streets, giving Peka Kroef a high degree of operational flexibility. From the buffer tank the water is pumped to the conditioning tanks where the pH of the wastewater is controlled. Wastewater is then pumped to the Biobed EGSB reactors where the COD conversion takes place. The conditioning tanks and the anaerobic reactors operate under 100 mbar pressure and are made from FRP. It is possible to operate without a gasholder or a compressor. In addition, the EGSB reactor guarantees operating under a "zero odor emission" and supports the aerobic post-treatment in order to increase nitrogen and phosphorus removal for final discharge to the sewer. Initial results of this Biobed reactor in the potato processing industry are very promising.

Case Study III. In this study, vegetable processing wastewaters were subjected to thermophilic treatment in UASB reactors at 55°C [46]. The high-strength wastewater streams, coming from steam peeling and balancing of carrot, potato, and swede were used. The

Figure 17 Schematic representation of the pretreatment stage and anaerobic treatment stage at Smiths Food (from Ref. 14).

Table 11 Performance Data of Wastewater Treatment Plant at Smiths Food (Source: Ref. 14)

Parameter__Unit Value Efficiency

Influent (data after primary clarifier)

Table 11 Performance Data of Wastewater Treatment Plant at Smiths Food (Source: Ref. 14)

Influent (data after primary clarifier)

Flow

m3/day

517

t-COD

mg/L

4566

s-COD

mg/L

2770

SS

mg/L

890

Anaerobic effluent

t-COD

mg/L

926

80%

s-COD

mg/L

266

90%

SS

mg/L

600

TKN

mg/L

196

Aerobic (final) effluent

t-COD

mg/L

165

96%

s-COD

mg/L

60

98%

BOD

mg/L

17

SS

mg/L

82

TKN

mg/L

4

wastewater characteristics are summarized in Table 12. Carbohydrates contributed 5060% of the COD in different wastewaters.

The reactors were inoculated with mesophilic granular sludge. Stable thermophilic methanogenesis with about 60% COD removal was reached within 28 days. During the 134 day study period, the loading rate was increased up to 24 kg COD/m3-day. High treatment efficiency of more than 90% COD removal and concomitant methane production of 7.3 m CH4/m3-day were achieved.

The highest specific methanogenic activity (SMA) reported in this study was 1.5 g CH4- COD/g VSS-day, while SMAS of 2.0 and 2.1 g COD/g VSS-day have been reported with sludge from 55°C UASB reactors treating other food industry wastewaters [52,53].

Key points of interest that can be drawn from this case study are as follows:

• The results support the previous finding that 55°C UASB reactors can be started with mesophilic granular sludge as inoculum.

• The anaerobic process performance was not affected by the changes in the wastewater due to the different processing vegetables.

• The achieved loading rates and COD removals demonstrated that the thermophilic high rate anaerobic process is a feasible method to treat hot and concentrated wastewaters from vegetable processing.

Design Example 6. Design an anaerobic process reactor to achieve 85% removal of COD from a preclarified wastewater flow 360 m3/day (95,100 gal/day) resulting from a potato factory, depending on the steam peeling method, where total influent COD=5000 mg/L, COD to be removed=85%, pH=6.2, and temperature=30°C. The anaerobic process parameters are: sludge age (SRT)=20 days (minimum), temperature=35°C, a=0.14 mg VSS/mg COD, 6=0.021 mg VSS/(mg VSS-day), K=0.0006 L/(mg VSS-day), Xv=5500 mg/L.

Figure 18 Schematic representation of the pretreatment stage and anaerobic treatment stage at Peka Kroef (from Ref. 14).

Table 12 Characteristics of Vegetable Processing Wastewaters After Removing Solids Through Settling and Drum

Total COD (g/L) Soluble COD (g/L)

Unit

Raw material Average Range

Average

Range

Steam peeling

Carrot

19.4 17.4-23.6

17.8

15.1-22.6

Potato

27.4 13.7-32.6

14.2

11.7-17.5

Blanching

Carrot

45.0 26.3-71.4

37.6

22.1-45.8

Potato

39.6 17.0-79.1

31.3

10.9-60.6

Swede

49.8 40.5-59.1

49.4

40.5-58.3

Solution: Prior to anaerobic treatment of potato processing wastewater, it is important to provide favorable conditions for the anaerobic process through equalization and neutralization of the influent. Because the preclarified wastewater is almost neutral, there is no need for neutralization, and accordingly no need for correction of pH and temperature. Buffering of the wastewater is necessary here, to guarantee constant or near-constant flow. Total daily flow (average)=360 m3/day. Flow (average after buffering)=15 m3/hour, assuming that retention time is approximately 1 day in the buffer tank (balancing tank), with volume=350 m3. Influent COD (average)=5000 mg/L. (Exact calculation of the buffer tank requires data plotted as the summation of inflow vs. time of day.)

Digester volume from the kinetic relationship:

Detention time: t =

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