The modern trickling filter typically includes the following major components: (1) rotary distributors with speed control; (2) modular plastic media (typically cross-flow media unless the bioreactor is treating high-strength wastewater, which warrants the use of vertical-flow media); (3) a mechanical aeration system (that consists of air distribution piping and low-pressure fans); (4) influent/recirculation pump station; and (5) covers that aid in the uniform distribution of air and foul air containment (for odor control). Covers may be equipped with sprinklers that can spray in-plant washwater to cool the media during emergency shut down periods. Trickling filter mechanics are poorly understood. Consequently, there is a general lack of mechanistic mathematical models and design approaches, and the design and operation of trickling filter and trickling filter/suspended growth (TF/SG) processes is empirical. Some empirical trickling filter design criteria are described in this paper. Benefits inherent to the trickling filter process (when compared with activated sludge processes) include operational simplicity, resistance to toxic and shock loads, and low energy requirements. However, trickling filters are susceptible to nuisance conditions that are primarily caused by macro fauna. Process mechanical components dedicated to minimizing the accumulation of macro fauna such as filter flies, worms, and snail (shells) are now standard. Unfortunately, information on the selection and design of these process components is fragmented and has been poorly documented. The trickling filter/solids contact process is the most common TF/SG process. This paper summarizes state-of-the art design and operational practice for the modern trickling filter. Water Environ.

Trickling filter and trickling filter-suspended growth process design and operation: a state-of-the-art review.

Simultaneous carbon and nitrogen removal with enhanced bioparticle circulation in a Circulating Fluidized Bed Biofilm Reactor

Mechanistic Models. Liu et al. (1998) experimentally and numerically demonstrated the existence of lag periods in biomass growth ranging from 0.3 to 2.5 hours under enriched culture substrate-unlimited conditions when growth switched from a rel atively long period of oxygen respiration to nitrate respiration. Magbanua and Bowers (1998) used a reduced activated-sludge process (ASP) model, including soluble substrate, floc-forming, and filamentous microorganisms to demonstrate that increasing the sludge recycle ratio from 0.5 to 2 reduced the substrate gradient in a plug-flow reactor and more than doubled the relative concentra tion of filamentous organisms, increasing the sludge bulking po tential. Lee et al. (1998) developed a steady-state ASP model that considered primary and co-metabolic biod?gradation, water sur face and air bubble volatilization, and adsorption for substrates with a broad properties range and proposed its use design of experiments for parameter evaluation. Nolasco et al. (1998) used the GPS-X model to design, and upon construction optimize, a pilot-scale, three-point step feed nitrogen and phosphorus removal process using online and labo ratory measurements. The default laboratory parameters were found robust for design purposes in predicting effluent soluble components. Optimal solids retention time (SRT) was 5 and 9 days at 25 to 30 and 10 to 14 ?C, respectively; optimal flow split produced equal food-to-microorganism ratios (F:Ms) in each stage; volatile fatty acid (VFA) addition of 50 mg/L as chemical oxygen demand (COD) was needed; and dissolved oxygen (DO) had to be approximately 1 mg/L at the end of each anaerobic compartment. Cinar et al. (1998) applied the International Asso ciation on Water Quality (IAWQ) activated sludge model 2 (ASM2) to characterize the nutrient removal of four ASPs. They concluded that the model was successful in modeling the plants having surface aeration and diffused aeration but could not char acterize the performance of the two oxidation ditch plants. Droste (1998) proposed modeling the endogenous heterotrophic decay process through an endogenous yield factor and estimated the associated parameters of the proposed model based on bioen ergetics theory. Kornaros and Lyberatos (1998) modelled the be havior of Pseudomonas denitrificans under transition among aer obic and anoxic conditions by using oxygen inhibition coefficients in their anoxic conditions model. Analyzing an industrial sequenc ing batch reactor (SBR) process, Coen et al. (1998) used tracer test simulation and the single-pass residence time distribution method to model each of three plug-flow reactors in parallel as two tanks in series with the first tank's volume being 5 to 20% of the second and each reactor having dead volume of 8 to 20%. Batch respiro grams exhibited three plateaus and thus three sets of Monod parameter pairs, some nonunique, were estimated. A two-parameter expression for the zone settling velocity versus solids concentration relationship was based on filtration theory that matched settling data of nonbiological suspensions (Islam and Karamisheva, 1998). Zheng and Bagley (1998) formulated a zone settling and compression clarifier model and predicted one-dimen sional calcium carbonate clarifier concentration profile data with r2 = 0.81 and activated-sludge concentration profile with r2 = 0.55. Chatellier and Audic (1998) improved the sludge blanket level predictive ability of a 61-layer, one-dimensional clarifier model when they corrected the measured batch settling velocity based on the measured SVI and the solids detention time in the

Modeling, instrumentation, automation, and optimization of wastewater treatment facilities.

The full-scale single-stage tertiary nitrifying trickling filter (NTF) at the Citrusdal Wastewater Treatment Plant provides for external nitrification of unclarified effluent from the facultative aerobic lagoon in order to meet standard effluent ammonia concentration requirements. The apparent ammonia nitrification rate (ApANR, gN/m2 media surface·d) of the NTF was sensitive to particulate organic loading rates which were predominantly in the form of algae, and the soluble COD removal rates increased under cold climates. Installation of forced-air ventilation fans improved the nitrification efficiency from 15% to 43%. An increase in hydraulic loading rate (HLR) by effluent recirculation significantly improved the ApANR, eradicated filter flies and decreased the prevalence of worms. Maximum ApANR of ~1.0 gN/m2·d was achieved yielding an ammonia- removal efficiency of approximately 71%. Profile samples collected along the NTF media depth indicated poor media wetting at low HLR resulting in low ApANR (<0.5 gN/m2·d). Also during the cold and rainy winter period, poor biofilm activity and prevalence of motile algae were observed, and under low hydraulic loading rates and warmer temperatures, a dominance of filter flies and fly larvae were observed. In contrast, in controlled laboratory studies, ApANRs up to 1.72 gN/m2·d (22.1 mgN/l removal) were attained, which, in conformity with full-scale, was also found to be sensitive to hydraulic loading conditions.

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Full-scale trials of external nitrification on plastic media nitrifying trickling filter

The performance of 10 cross flow structured plastic media trickling filter plants is discussed. The sites are arranged in a number of configurations with the plastic media filters operating as sole secondary process units, in parallel with other process units or in double filtration configuration. Data for 2003 and 2004 show that the plants are performing well. Data are presented for the plastic media filter stream where it is available. Two operational problems are discussed; overgrazing of nitrifying biofilms by excessive populations of snails and loss of nitrification performance in cold weather, particularly on smaller sites. Overgrazing by snails, identified as Lymnaea peregra has been controlled on one site by the annual treatment of an isolated filter with high ammoniacal nitrogen strength sludge dewatering liquors. On a second site suffering a similar problem the plastic media filter duty was changed from single filtration to being the primary filters in a double filtration system with existing stone media filters used for secondary filters. Loss of nitrification performance has been associated with excessive heat loss on some sites. Filters built as tower structures, with typically 4–5 m media depth allow easy control of airflow via restriction of the engineered ventilation openings at the base of the filter. This has been found to have a direct impact on the degree of cooling. However, airflow control on plastic media retrofitted into wide shallow stone media filter shells with 1.6–2 m media depth is more difficult. These structures offer less opportunity for control of airflow and associated heat losses.

Rick L. Chan

Papers

Escargot... Going... Gone A Nevada facility controls snails with centrate to meet stringent total nitrogen limits

The use of ammonia to control snails in trickling filters

The coupling of two biological phosphorus removal processes to increase operational flexibility and improve performance

Can Higher Loadings Reduce Secondary Sludge Yields from Trickling Filter/Activated Sludge Plants?