improved biological nitrification using recirculation in ...

Wat. Sci. Tech. Vol. 26,No. 3-4, pp. 545-553,1992.

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IMPROVED BIOLOGICAL NITRIFICATION USING RECIRCULATION IN ROTATING BIOLOGICAL CONTACTORS R. Klees* and 1. Silverstein** *Bureau of Science and Technology, U.S. Agency for International Development, Washington, DC 20523-1817, USA **Department of Civil, Architecwral and Environmental Engineering, University of Colorado, Boulder, CO 80209, USA

ABSTRACT The effect of recirculation on biological nitrification in rotating biological contactors (RBCs) was examined in an on-site pilot-scale RBC system which simulated full-scale plant RBC operations at the Longmont, Colorado,Wastewater Treatment Plant (LWWTP). Two LWWTP wastewater streams were used as pilot RBC influent. One wastewater stream, designated the "fresh" influent, was first stage trickling filter effluent. The second stream, designated the "recirculation" influent, was clarified secondary effluent. Three recirculation ratios, defined as the ratio of the recirculation influent to the fresh influent, were examined: 0 (no recirculation), 1 (50% of the flow is recirculation), and 3 (75% of the flow is recirculation). Each recirculation ratio was studied at three hydraulic loading rates. Recirculation improved nitrification at all hydraulic loading rates. Improved nitrification with recirculation was due to the dilution of influent biodegradable organic carbon (BOD5) which occurs as a result of mixing secondary plant effluent with first stage effluent. An unexpected and operationally significant result was that extremely low concentrations of influent organic carbon did not improve nitrification. KEYWORDS Biological nitrification; biofilm; nutrient removal; rotating biological contactor; organic carbon; fixed-film; wastewater treatment; recirculation

I NTRODUCTIO N

Problem Statement Ammonia nitrogen is not easy to remove by conventional wastewater treatment techniques and frequently remains in the wastewater treatment plant effluent. The adverse environmental impacts associated with ammonia nitrogen include promotion of eutrophication; toxicity to aquatic organisms; and depletion of dissolved oxygen in receiving streams due to bacterial oxidation of ammonia to nitrate. Because of these impacts,the discharge of ammonia nitrogen is increasingly being restricted. Stringent limitations on ammonia nitrogen discharge, while necessary for pollution control, can exceed the ability of traditional wastewater treatment plants to produce high quality effluents at reasonable costs. Consequently, economical and innovative wastewater treatment techniques are needed to meet the demands for ammonia nitrogen control. 545

R. KLEES andJ. SILVERSTEIN

546

Biological Nitrification Biological nitrification, the most widely accepted ammonia removal process in wastewater treatment, is a microbial process in which ammonia nitrogen is oxidized to nitrate by two groups of bacteria which operate in sequence. Nitrifying bacteria (autotrophs) use ammonia nitrogen as an energy source, inorganic carbon as a carbon source, and oxygen as the terminal electron acceptor. Heterotrophs, other bacteria also present in wastewater, use organic carbon for a carbon and energy source and oxygen as the terminal electron acceptor. Several microbiological factors are important to note in order to understand and control for nitrification in a RBC. First,while both autotrophs and heterotrophs require oxygen,nitrifying bacteria need three to four times more oxygen than heterotrophs. Second, the growth rate of nitrifying bacteria is at least an order of magnitude less than heterotrophs (Barnes and Bliss, 1983). Finally, the half velocity constant, K" for organic material present in wastewater is relatively high (50 mg/l BODs) compared to the K. for nitrifying autotrophs (5 mg/l NH3-N) (Barnes and Bliss,1983). These factors in concert in a biofilm process (such as RBCs) may result in the limitation of nitrification by the small growth rate of autotrophs which can be quickly submerged beyond the limit of oxygen penetration in a fast growing heterotrophic biofilm. Experience gained at full-scale RBCs (Brenner, 1984) and pilot-scale RBC studies (Chung and Borchardt, 1984) have shown that nitrifying bacteria cannot compete effectively until organic carbon is less than 15 mg/l BODs·

Project Background 3 The LWWTP treats 30,280 m /day (8MGD) of municipal wastewater in a process which includes an equalization basin; primary clarification; first stage trickling filtration; a second stage treatment process of both RBCs and trickling filters; solids contact; secondary clarification; chlorination; and dechlorination. The plant aims to maintain the highest possible hydraulic loading to the RBCs which can reliably remove sufficent ammonia nitrogen to comply with the discharge permit. Prior to this study,the LWWTP achieved some nitrification in its second stage RBCs and trickling filters, but performance was inconsistent and uncontrollable since the process control factors were not understood. It was known that the influent to the RBCs (first stage trickling filter effluent) contained an organic carbon concentration within the range found to be inhibitory to nitrification. Therefore, a method of reducing the organic carbon in the influent to the RBCs was needed in order to optimize nitrification. Recirculation is the process of returning effluent to the inlet of a treatment process and passing the wastewater through the unit again. The LWWTP flow scheme allowed for recirculation of clarified second stage plant effluent to the RBCs. Although recirculation is a common wastewater treatment procedure in trickling filter operations,it has never been applied to full-scale RBCs. However,it was hypothesized that this recirculated stream could be used successfully to dilute the RBC influent organic carbon and hence increase nitrification. The research goals of the project as reported in this paper were 1) to determine the effect of recirculation on nitrification in RBCs and 2) to develop an operational scheme for reliable ammonia removal using existing LWWTP equipment. Other research goals and results are presented elsewhere (Klees, 1989).

MATERIALS AND METHODS

RBC Pilot-Scale System

A pilot-scale RBC system was constructed at the LWWTP using two commercial RBC pilot units manufactured by the Envirex Corporation. Each RBC unit consisted of a tank; plastic disks on a rotating shaft; a chain drive and motor. The tank had dimension of 1.8 m (72") len�th, 0.7 m (28") width and 0.6 m (24") height. The tank was divided by bulkheads into an influent chamber of 191 (5 gal) volume and four equal volume tanks (stages) of 33 I (8.7 gal) each. A salt tracer study was performed to determine the

Biological nitrification

547

mixing pattern of the tank and confirmed that the RBC unit behaves hydraulically as a series of four ideal continuous-stirred tank reactors. The tank contained light-weight polyethylene 0.5 m ( 19.5") disks with 23.2 m2 (250 ft) of surface area for biological growth. The disks were mounted on a shaft which rotated at 4 rpm providing a peripheral velocity of 6.0 mlmin ( 19.5 ft/min). The two RBC pilot units were operated in parallel in a heated building similar to the enclosed full-scale RBC units. The units were operated continuously during the experimental period - February 1988 to June 1989. Influent Wastewater LWWTP wastewater from two sources was used for the pilot RBC experiments. The pilot RBC influent wastewater stream designated as "fresh" was LWWTP unclarified first stage trickling filter effluent. This is the wastewater stream traditionally applied to the full-scale RBCs in Longmont. The pilot RBC influent wastewater stream designated as "recirculation" was LWWTP clarified secondary effluent. The two wastewater streams were pumped continuously to separate reservoirs from which the flow to the RBCs was controlled by peristaltic pumps. The range of influent wastewater characteristics for the experiments reported in this paper are shown in Table 1. As the LWWTP began to nitrify, influent ammonia decreased with recirculation and supplementary ammonia was spiked in the pilot RBC to avoid substrate limiting conditions. Note that the environmental factors of concern in nitrification (pH, alkalinity, and DO) were sufficient for nitrification. Temperature was found to have a minimal effect on nitrification within the range used in these experiments, 13 - 2 1°C. TABLE 1 RBC Influent Wastewater Characteristics Parameter

Range

pH

7.2-7.9

Alkalinity Temperature

146-211 mg/I CaCO, 13.OC-21.4°C

DO

5.5-6.8 (mg/I)

TOC

9.7-19.5 (mg/I)

sBODs

9.3-32.3 (mg/I)

tBODs

29.0-76.1 (mg/\)

SS

15.9-140.2 (mg/\)

NH3-N

8.4-34.4 (mg/I)

Experimental Design Three recirculation ratios, defined as the ratio of the recirculation stream to the fresh stream, were examined: o (no recirculation), 1 (50% of the RBC influent flow is recirculation, and 3 (75% of the RBC influent flow is recirculation). Each recirculation ratio was studied at three hydraulic loading rates for a total of nine experiments. The hydraulic loading rates of 0.082 m3jm2/day (2 gpd/ft), 0.164 m3/m2/day (4 gpd/ft), and 0.245 m3/m2/day (6 gpd/ft), with corresponding hydraulic detention times of 101 minutes, 51 minutes, and 33 minutes, respectively, are referred to as the low, intermediate and high hydraulic loading rate experiments throughout this paper. The intermediate and high hydraulic loading rates are higher than those recommended by RBC manufacturers or covered in design curves but represent realistic operating conditions at the LWWTP.

Monitoring and Sampling Procedures The RBC system was determined to be in steady-state when the concentrations of influent and effluent

548

R. KLEES

and J.

SIT..VERS1EIN

ammonia nitrogen were constant. When steady-state was reached, samples were collected for 5-6 days. Throughout the study period, the two RBCs behaved identically so the results of the sampling were pooled for a total of 10-12 data points. The RBCs were sampled for pH, temperature, dissolved oxygen (DO), alkalinity, ammonia nitrogen, nitrite-nitrate nitrogen, total organic carbon (TOC), soluble and total BODs , and suspended solids. Flow was monitored daily. Biofilm was measured for thickness and density. Visual and microscopic biofilm observations were made.

Laboratory Analysis All wastewater chemical analyses were performed according to Standard Methods (American Public Health Association,1985). A scraping technique adapted from Chung and Borchardt (1984) was used to measure biofilm thickness and density and is described elsewhere (Klees, 1989).

RESULTS A ND DISCUSSIO N As predicted, recirculation improved nitrification at all hydraulic loading rates, as can be seen in Figure 1. 26 24

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