Evaluation of quicklime incorporation in bench-scale and full-scale lime stabilized biosolids using a flat surface pH electrode.

ABSTRACT

Uniform lime incorporation into sewage sludge is critical for biosolid lime stabilization processes. There is no class B biosolids regulation for lime incorporation. The slurry method is currently used to evaluate the pH of limed biosolids, but this method homogenizes the biosolids and potentially masks poor lime mixing. In this study, a flat-surface pH electrode was used in bench-scale and full-scale experiments to measure the pH of lime-stabilized biosolids without creating slurries. The standard deviation of 15 pH measurements at different locations in a biosolid sample was used to assess mixing quality. The bench-scale experimental study showed that well-mixed limed biosolids had consistently high pHs (~12) with low standard deviations (<0.5 pH units), whereas poorly mixed biosolids had areas with low pH (<10) and high standard deviations (>2 pH units). Poorly mixed biosolids exhibited rapid and marked pH reduction, as well as offensive odor generation, whereas well-mixed biosolids resisted pH reduction and offensive odor generation. The full-scale study aimed at improving lime incorporation and biosolids quality confirmed the use of a flat surface pH electrode to capture low pH regions in biosolids that were masked by the current slurry method.

INTRODUCTION

Lime stabilization is a U.S. Environmental Protection Agency (EPA)-approved process used at approximately 20% of wastewater treatment plants in the United States to significantly reduce pathogens in sewage sludge and to control odors. (1,2) Inactivation of microorganisms and subsequent reduction in microbially produced odorants result from the increased pH caused by the low-dose addition of lime (CaO or Ca[OH][.sub.2]) to sewage sludge. The increased pH further reduces offensive odors by inhibiting the release of sulfur-based odorants (hydrogen sulfide). (3) However, the release of nitrogen-based odorants has been reported as a major odor problem at elevated pHs. (3,4) Offensive odor has become one of the predominant factors affecting public perception of biosolids land application. (5)

The effectiveness of lime stabilization for pathogen reduction and odor control depends on achieving and maintaining a high pH (>12) through sufficient lime addition and incorporation. (2) Intimate mixing of lime and sewage sludge is important for eliminating regions of low pH within the biosolids. (6) Poor lime incorporation results in inadequately stabilized regions of biosolids that lead to microbial regrowth, which produces organic acids and C[O.sub.2], and drives pH reduction during biosolids storage. (7,8) Paulsrud and Eikum (7) reported a considerable odor generation as the pH of lime stabilized biosolids fell below 11. In addition, pathogen regrowth may occur as a result of pH reduction. (9)

The recommended measurements for the stability of lime stabilized biosolids are pH and pH change during storage. (9) Currently, regulatory compliance for class B biosolids is based on achieving the operation conditions of a pH above 12 for 2 hr for pathogen reduction and a pH above 11.5 for an additional 22 hr for vector attraction reduction. (2) However, class B biosolids regulations do not require the retesting of biosolids after extended storage, (2) which cannot reflect the potential pH reduction and odor problems, because improperly lime-stabilized biosolids can regrow pathogens and become very odorous.

The current method for determining biosolids pH is to mix 10 g of biosolids and water at a 1:2 ratio and measure the pH of the resulting slurry. (2) The disadvantage of the slurry method is that the creation of a slurry homogenizes the biosolids and ensures that all of the lime has reacted, thus masking regions with poor lime incorporation and producing a false sense of stability. Burnham et al. (10) used the standard slurry method to measure the pH of large limed sludge samples, as well as 48 randomly selected 0.2-g samples of lime-stabilized biosolids dosed with 20-22% lime (dry wt basis) from a full-scale treatment process. Although all of the larger samples had pH values above 12, only 5 of the 48 0.2-g samples had pH values above 12. Five of the 0.2-g samples had pH values between 10 and 11.9, whereas the remaining 38 samples had pH values less than 10.

Calcium content can also be measured to indicate mixing quality. North (11) assessed the mixing quality of bench-scale treated, lime-stabilized biosolids by measuring the calcium concentration at various locations throughout a biosolid sample. However, using calcium measurements to assess mixing quality is time consuming and may require contracting to an outside laboratory. Furthermore, it does not provide related information for the operational conditions to meet the required pH in biosolids treatment.

There are several factors affecting lime incorporation in biosolids treatment. North (11) found that a longer mixing time resulted in a more uniform distribution of lime and a greater reduction in fecal coliform. The increased mixing time allows for more opportunities for lime to contact the sewage sludge. However, mixing is not solely a function of time; the intensity of the mixing must be strong enough to shear biosolid masses and allow lime to contact inner biosolids regions.

There is a great need for a rapid and inexpensive method that can be performed on-site to assess the mixing quality of lime-stabilized biosolids. Breitenbeck and Bremmer (12) and Adamchuk et al. (13) demonstrated the use of a flat-surface pH electrode (Sensorex 450C), with reproducible results, for the measurement of soil samples at low moisture content without creating a slurry. There is limited information available for using a flat-surface pH electrode to evaluate the efficiency of lime incorporation in biosolids and relating the pH values with odor generation. Therefore, the objectives of this study were to apply a flat surface pH electrode to assess mixing quality of lime-stabilized biosolids and to establish a correlation of pH, lime dosages, and related odor generation in bench-scale and full-scale experiments.

EXPERIMENTAL WORK

Wastewater Treatment Plant

A Pennsylvania wastewater treatment plant with a daily flow of 7.5 million gal and a daily production of 64 wet t of lime-stabilized biosolids was selected based on strong odors, which were caused by poor lime incorporation. Primary and secondary waste-activated sludge are combined and dewatered to 25-27% solids using a high-rate centrifuge. The dewatered sludge is augured horizontally for 28 ft and then on an incline for 32 ft into a pug mill mixer (Figure la). At this point, quick lime (CaO) is dosed at approximately 3.5% (wet weight basis of sludge grams per gram) and incorporated into the sludge via a pug mill mixer. The lime-stabilized biosolids (32% solids) are conveyed horizontally for an additional 18 ft before falling into the bed of a transport truck for ultimate land application.

Because dewatered sludge is conveyed 60 ft before lime addition, large sludge masses are generated, requiring additional mixing to thoroughly incorporate the lime. However, the existing mixing equipment does not provide sufficient time or force to thoroughly incorporate the lime into these larger masses.

Bench-Scale Mixing Experiments

Lime-stabilized biosolids were collected from the treatment plant to serve as the control sample. In addition, dewatered and unlimed sludge with total solids of 27% was collected from the plant and dosed with quicklime at rates of 3.5%, 7%, and 10% (wet weight basis of sludge grams per gram) to obtain an individual sample weight of 2 kg (wet weight). Sludge and lime were mixed in a Reynolds Chef II (Reynolds Electric Company) dough mixer for 60 sec on slow speed using a slotted paddle mixing attachment. The quicklime used in the study was collected from the lime silo at the treatment plant. The mixed products were stored in 2-gal high-density polyethylene (HDPE) resealable containers for 28 days at 19-22 [degrees]C. During the storage period, a portion of biosolids was removed from the containers on a weekly basis for pH and odor measurements.

Full-Scale Lime Addition Point Modification

Augers can be used to simultaneously convey and mix biosolids. (11) Therefore, to increase mixing and improve biosolids quality of the full-scale biosolids treatment process, the lime addition point was temporarily relocated to use the mixing potential of 25 ft of existing inclined auger (Figure lb). In addition to providing preliminary mixing before reaching the pug mill, this modification results in the addition of lime to smaller sludge masses, which will make it easier to incorporate the lime. A box constructed out of 0.25-in. plywood and fitted with a plexiglas window was used to replace a section of covering over the auger shaft. The box allowed for continued monitoring of the lime addition without causing the dispersion of lime dust. The lime was conveyed down to the experimental addition point using a 6.5-in. diameter lime trough (Figure lb) to which three small electric motors with unbalanced drive shafts were attached at the bottom, the middle, and the top of the lime trough to provide a vibration to facilitate the lime movement.