TREATMENT TECHNOLOGIES FOR FOOD PROCESSING WASTEWATER

Patrick W. Given, M.Sc., P.Eng.
Associated Engineering
Edmonton, Alberta

Barry Vallance, P.Eng.
Associated Engineering
Calgary, Alberta

Jan With
Rogers Sugar Ltd.
Vancouver, B.C.

Contact: givenp@ae.ca

ABSTRACT

This paper reviews advanced technologies for treating wastewaters from food processing industries, in general, and sugarbeet processing facilities, in particular. The review composed an integral part of Rogers Sugar’s plans to expand and upgrade its Taber factory in southern Alberta. The wastewater component of the project first addressed a new, stringent wastewater discharge bylaw which required the factory to implement pretreatment. Subsequently, following detailed study, Rogers Sugar elected to go beyond the bylaw requirements, and construct a combination of anaerobic and aerobic wastewater treatment facilities which would meet more stringent standards, enabling direct stream discharge.

Reviewed anaerobic treatment technologies include anaerobic contact, anaerobic fluidized bed, and upflow anaerobic sludge blanket reactors. Reviewed aerobic treatment technology focuses on activated sludge with nitrification-denitrification. Several facilities in the United States and Europe were visited to assist in the technology review.

The technology reviews were paralleled by performance specifications and bid invitations to vendors with recognized experience and expertise in designing and supplying facilities for treating sugarbeet wastewater. Currently, the advanced anaerobic-aerobic wastewater facilities are undergoing commissioning. The plant is designed to remove in excess of 99% of incoming BOD loads and to nitrify-denitrify.

INTRODUCTION

Food processing industries utilize physical, chemical, and biological processes to treat their wastewater streams. The specific processes and combinations thereof reflect industry preferences, experiences and site-specific conditions.

This paper focuses on advanced, biological treatment technologies for high-strength wastewater from sugarbeet factories. The treatment technologies for sugarbeet wastewater also have broader applicability, with appropriate design loading and other modifications, to many other food processing wastewaters. Since sugarbeet wastewater not only exhibits high strength (BOD and COD well in excess of 1,000 mg/L), but also is readily biodegradable, it is well suited to anaerobic treatment. Aerobic treatment for effluent polishing is also practised at some facilities.

Before exploring the details of the technologies, a brief description of wastewater sources and characteristics, as well as waste management approaches, is presented.

WASTEWATER SOURCES, CHARACTERISTICS AND MANAGEMENT APPROACHES

Sugarbeet factories typically generate two main wastewater sources: flume wastes and condensate wastes. In addition, factories have cooling water circuits which generally include cooling towers or cooling ponds and recycle. Fig. 1 illustrates a simplified flow diagram with one of several in-plant waste management techniques – anaerobic treatment of flume system wastewater and aerobic treatment of anaerobic effluent along with condensate.

Most sugarbeet factories utilize a flume system for transporting and preliminary cleaning of the beets. Even with fresh sugarbeets, some sugar leaches from the beets contributing to increased organic loads in the flume stream. This load can continue increasing, say from a few hundred mg/L BOD to more than 20,000 mg/L BOD, with recycle of clarified flume water and deterioration of beets during storage. The ready source of soluble food in the flume water provides an ideal substrate for bacterial growth which must be kept under control. Lime often is fed for corrosion control and for bacterial control. This application of lime contributes to characteristic high calcium concentrations, particularly in North American sugarbeet factories. Some European factories have managed to reduce the need for lime; this has been achieved largely through a combination of a shorter processing season (reduced beet spoilage) and treated effluent recycle (increased alkalinity to provide buffering capacity in the flume water).

Condensate wastewater originates from the evaporation processes utilized to concentrate sugar from the sugarbeets. Water comprises approximately 75% of the sugarbeet weight. Theoretically, this condensate water equates to the total weight of water removed from processed beets. Relative to domestic wastewater, condensate may exhibit high ammonia concentration, moderate BOD, and low alkalinity and phosphorus. Table 1 summarizes typical raw wastewater characteristics at Rogers Sugar, Taber Factory.

Table 1. Raw Wastewater Characteristics at Rogers Sugar

As previously indicated, wastewater recycle provides one excellent method of managing wastes. Other methods include pretreatment for municipal discharge and advanced treatment for stream discharge or effluent irrigation.

Wastewater Recycle

Plant flume wastewater recycle options may include:

With recycle of anaerobic effluent to the flume system, pre-aeration may be necessary for odour control.

Plant condensate utilization and recycle options may include:

Pretreatment for Municipal Discharge

Prior to its recent expansion and upgrading, the Taber Factory discharged mud pond supernatant to the Town of Taber’s industrial lagoon system. Rogers Sugar has been one of the principal waste contributors to the Town’s system (aerated and storage ponds). In view of increasing concerns with the ability of the lagoons to provide adequate treatment, the Town developed a sewage discharge bylaw applicable to all food processing discharges served by the Town. Rogers Sugar decided to go beyond the bylaw requirements and meet more stringent limits, enabling direct stream discharge.

Complete Treatment for Stream Discharge

Alberta Environment set stringent limits for stream discharges. To meet the limits, a combination of high-efficiency anaerobic treatment and aerobic treatment became necessary.

The following sections describe anaerobic and aerobic treatment technologies reviewed by Rogers Sugar and Associated Engineering.

ANAEROBIC TREATMENT TECHNOLOGIES

Rogers Sugar and Associated Engineering representatives visited a number of sugarbeet factory wastewater treatment facilities to help assess appropriate wastewater treatment processes offered by various vendors. The facilities were located in the United States, Netherlands, Germany, Denmark and Belgium. Two main anaerobic treatment technologies were prevalent: upflow anaerobic sludge blanket (UASB) and anaerobic contact (AC). In addition, one German facility used an anaerobic fluidized bed (AFB) reactor which, although suited to various industrial wastewaters, is used at only one sugarbeet factory.

Anaerobic Contact Process

The anaerobic contact (AC) process is analogous to the conventional activated sludge (AS) process in that the active solids mass necessary for biological stabilization of the incoming organics is settled in a separate clarifier and recycled back to the reactor. Of course, the primary difference between the AC and AS process is that the former occurs under anaerobic conditions, whereas the latter occurs principally under aerobic conditions. The AC process is suited to treating high strength, readily biodegradable, soluble organic wastes. The anaerobic fluidized bed (AFB) and upflow anaerobic sludge blanket (UASB) processes described below are also suitable for such waste streams.

The AC process has found favour in the treatment of sugarbeet wastewaters where calcium concentrations are high (greater than 600 mg/L). Internal calcium carbonate fouling of AC reactor components is less of a concern than with AFB and UASB processes. However, it is noted that the latter processes have been used successfully for sugarbeet factory wastewater treatment; generally, in these cases, steps have been taken to minimize calcium concentrations in the waste streams.

Anaerobic Fluidized Bed Process

The anaerobic fluidized bed process incorporates high rate anaerobic treatment in a single reactor. An inert carrier material is utilized as a nucleus around which anaerobic organisms attach and grow while feeding on wastewater organics. The carrier material also provides weight to the organic mass to prevent its loss from the system which would otherwise occur because of high upflow or fluidization velocities.

Although there are several industries utilizing the AFB process, only one known sugarbeet factory utilizes the process. The sugarbeet factory at Clauen, Germany has a single AFB reactor treating mud pond supernatant wastewater during and immediately following the sugarbeet processing campaign. Anaerobically treated effluent is recycled as make-up water to the flume water circuit. In addition, aerobic treatment facilities provide treatment of condensate during the campaign and treatment of anaerobic effluent following the campaign.

The Clauen AFB reactor is a cylindrical steel tower about 6 m diameter and 30 m high. The upper portion of the tower enlarges to more than 9 m diameter where fixed, proprietary equipment provides solids-liquid-gas separation. High effluent recirculation rates, resulting in upflow velocities of 15 m/h are employed to suspend pumice carrier material (300 m³). To minimize carry-over or loss of the pumice, effective rise rates in the upper portion of the tower are reduced by the diameter enlargement and by plate settlers. The system treats mud pond supernatant flows of 60 to 130 m³/h with COD loads up to 25 t/d and calcium concentrations around 1,100 mg/L. As of 1998, the system was operating at 12.5 t/d COD load and was reported by the operators to achieve 95% COD removal. Anaerobically treated water is returned as make-up water to the flume water circuit. At the end of the sugarbeet campaign each year, the anaerobic effluent is treated aerobically before discharge instead of being recycled to the flume water circuit. Excess solids including pumice material with calcium carbonate deposits are wasted from the bottom of the tower as required. The loss of spent carrier material is compensated by the addition of approximately 1 to 1.5 m³ of fresh pumice per day.

Upflow Anaerobic Sludge Blanket Process

The UASB process also relies on upflow velocities to suspend anaerobic solids. However, upflow velocities are much lower than utilized in the AFB process since no carrier material is provided. Instead the anaerobic active mass tends to form a granular type of sludge which assists with its retention in the anaerobic reactor.

Various differences between UASB reactors were noted on visits to various European facilities. Some reactors are constructed of concrete; others of steel. Some reactors are rectangular; others are cylindrical. Some are moderate height (6 m ); one extended 20 m high. The reactors generally had various proprietary designs for distributing influent and recirculated flows at the bottom as well as three-phase (effluent, solids, gas) separators at the top.

Table 2 summarizes some of the main differences between three anaerobic treatment technologies: the AC process, the AFB process and the UASB process.

Table 2. Comparison of Three Anaerobic Processes

Besides the main process differences, significant differences also occur between various vendors of a particular process. For example, some UASBs are cylindrical; others are rectangular. As well, UASB reactor internals vary significantly between vendors in terms of influent distribution, effluent collection, solids retention and gas collection. With respect to AC reactors, some utilize a separate hydrolysis tank ahead of the main anaerobic reactor; others do not. Some use gas mixers; others use mechanical mixers (top- or side-mount configurations). Some use lamella clarifiers for settling and returning the anaerobic sludge; others use conventional circular clarifiers. However, regardless of the design differences and preferences exhibited by the various vendors, all systems are reported to achieve significant COD removals, normally in the 85-90% range. Also, BOD removals are slightly higher.

All facilities utilize various chemical feeds for process enhancements. Generally, phosphoric acid is fed to provide the proper nutrient balance. Other chemical feeds may include anti-foam to control reactor foam, antiscalants to reduce calcium carbonate precipitation, and weak acids or chlorine to clean heat exchangers. Following the processing season, facilities sometimes utilized nitrogen gas to purge methane from off-gas collection lines. In summary, different facilities successfully utilize various combinations of chemicals to help provide effective anaerobic treatment.

AEROBIC TREATMENT TECHNOLOGIES

This discussion is restricted to activated sludge with nitrification-denitrification. Other aerobic treatment technologies (e.g. facultative lagoons, aerated lagoons, trickling filters, rotating biological contactors, oxidation ditches, sequencing batch reactors) could also have merit in specific instances.

Some factories utilize aerobic facilities for further treatment of anaerobic effluent and/or treatment of condensates. The activated sludge process with nitrification-denitrification appears to be the aerobic treatment system of choice at sugarbeet factories. Typically at these facilities, a small flow of raw, high-strength wastewater is utilized as a carbon source for proper denitrification.

The most common nitrification-denitrification process configuration includes concentric tanks with a centre anoxic zone and an outer aerated zone. Coarse bubble diffused aeration systems predominate. However, both mechanical surface aeration and jet aeration provide process air and mixing at some facilities. High recirculation ratios, between 4:1 and 10:1 (recirculation flow:plant flow), provide nitrified mixed liquor return to the anoxic zone where denitrification occurs.

Circular secondary clarifiers follow the aeration systems. Some designs also incorporate a deaeration chamber between the aeration tank and clarifier. Although not observed during visits to various facilities, rectangular clarifiers or even no clarifiers (with sequencing batch reactors) should provide other solids separation options.

WASTEWATER TREATMENT IMPLEMENTATION AT ROGERS SUGAR

Preselection of Technologies and Vendor

Six experienced vendors of treatment equipment were invited to submit bids to provide both anaerobic and aerobic treatment equipment. Performance specifications, performance guarantees, and materials standards were detailed in the specification package. Of the six vendors, four submitted bonafide bids that were analyzed not only in terms of equipment costs, but also in terms of overall capital and operating costs. A key factor in the selection process involved the vendor’s proposed methods for dealing with identified concerns about high calcium concentrations in the wastewater.

A novel component of the overall design involves utilization of the existing mud pond to provide organic surge capacity for late campaign processing periods when waste strengths can rise appreciably. A flow and mass balance analysis assisted in estimating waste load accumulations in the mud pond with various reactor feed rates. The analysis showed that utilization of the available pond storage capacity resulted in nearly 70% reduction in the required anaerobic reactor size. As a result, significant savings in required capital expenditures were realized.

The selected vendor’s design incorporates a rectangular UASB reactor followed by a circular anoxic tank, a circular aeration tank, and a circular secondary clarifier. All tanks are constructed of reinforced concrete. The design of the anaerobic reactor also accommodates the historic calcium loading of the wastewater, principally through influent dilution with condensate. In addition, Rogers Sugar plans to recycle anaerobically treated effluent (following pre-aeration) to help minimize lime use. Recycled effluent will provide additional alkalinity to improve the stability of flume water pH.

The temperature of the anaerobic reactor is controlled through a combination of up to three influent heating methods: direct addition of hot condensate, heat recovery by cross exchange with the anaerobic effluent, and direct sparging of steam from a dedicated boiler into the influent. The latter method is required during the post campaign period when hot condensate is not available.

Currently, off-gas from the anaerobic reactor is collected and flared. During the post campaign period, approximately one-third of the off-gas will be used to fire the dedicated steam boiler. If operating experience confirms the estimated gas production, Rogers Sugar will consider utilizing the excess gas as a supplemental feed to the factory boilers.

Project Implementation

The project involved several challenges regarding the execution of the design and construction of the facility. In order to meet a September 1, 1999 deadline for facility start-up, the detailed design and construction required completion in less than one year. To meet this requirement, the project was executed on a fast track, EPCM basis with several construction and equipment supply contracts. These included:

The process design and equipment supply contract included a performance guarantee and responsibility for operating the facility during the first operating season.

To ensure the project was successful, significant effort and emphasis was placed on project implementation planning. A detailed project implementation plan was developed with input from all project stakeholders and team members to clearly define the project goals, constraints, roles and responsibilities. The project was completed on time and within budget.

SUMMARY

Several anaerobic treatment technologies are suited to treatment of high-strength wastewater, providing BOD removals exceeding 90%. In combination with aerobic treatment facilities, the technologies can provide in excess of 99% BOD removal and significant nitrification-denitrification.

The selected wastewater treatment system at Rogers Sugar will incorporate UASB treatment followed by activated sludge nitrification-denitrification treatment. The types of advanced wastewater treatment systems at the Taber factory also could have application at other locations with similar, high-strength wastes.