Resources, Conservation and Recycling, 11 ( 1994 ) 149-159 149 Elsevier Science B.V.

A comparison of biological nutrient removal in intermittent cyclic and continuous activated

sludge systems

Kin-man Ho, Paul F. Greenfield, Linda L. Blackall, Peter R.F. Bell and Andre Krol

Department of Chemical Engineering, University of Queensland, St. Lucia, Queensland, Australia

ABSTRACT

An assessment was made of biological phosphorus and nitrogen removal and related sludge bulking control in both intermittent cyclic and continuous activated sludge systems. Experimental results from a laboratory-scale investigation using fermented domestic sewage indicated the advantages of achiev- ing high levels of P and N removal and good sludge-settling properties using the intermittent cyclic process, as compared to the continuous process. The modified intermittently fed and decanted system incorporating non-mixing sequences produced an effluent quality of NO3-N < 5 mg L- J, PO4-P < 1 mg L -~ and NH3-N< 2 mg L -~, with a non-bulking sludge having a sludge volume index < 120 mL g- 1, despite unfavourable influent characteristics (TCOD: TKN < 7, TKN > 60 mg L- 1, TP-9 mg L- and RBCOD < 30 mg L - ' ). In contrast, the modified 4-stage Bardenpho process could not achieve effluent NO3-N and PO4-P concentrations of < 10 and < 1 mg L- l, respectively. The sludge volume index (SVI) values were always higher than 250 mL g-~ and occasionally reached as high as 422 mL g-~. The causative filaments were low F: M growers - Types 0041/0675.

I N T R O D U C T I O N

Since the first success in achieving biological P removal in a continuous full-scale biological N removal plant by Barnard in 1976 (Barnard, 1988), incorporation of biological P removal in a biological N removal plant is con- sidered to be a generally achievable objective. Design and operation of bio- logical nutrient removal (BNR) plants are now required not only to optimise these two parallel but interactive processes to maximise both nitrogen and phosphorus removal but also to simultaneously control the associated sludge bulking problems resulted from the proliferation of filamentous bacteria.

While biological P removal requires alternating anaerobic and aerobic re- action conditions for the respective anaerobic readily biodegradable chemical oxygen demand (RBCOD) accumulation/P release and subsequent aerobic

Correspondence to: Kin-man Ho, Depar tment of Chemical Engineering, Universi ty of Queens- land, St. Lucia, Qld. 4072, Australia.

0921-3449/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved. SSDI 0 9 2 1 - 3 4 4 9 ( 9 3 ) E 0 0 5 7 - W

1 5 0 K.-m. HO ET AL.

P uptake (or the less energy-efficient anoxic P uptake (Vlekke et al., 1988 ) ) by the biological P removal (BPR) bacteria, biological N removal relies on nitrification/denitrification. The influent ammonia together with that hydro- lysed from organic nitrogen is converted to nitrite, then to nitrate by the au- totrophs, Nitrosomonas spp. and Nitrobacter spp., respectively, under aerobic conditions. The nitrate produced is subsequently reduced to atmospheric ni- trogen gas by facultative heterotrophs under anoxic conditions. Anaerobic conditions imply an absence of exogenous electron acceptors - dissolved ox- ygen (DO) and nitrate, while anoxic conditions imply the absence of DO only.

The BPR organisms can only sequester RBCOD under strict anaerobic conditions; in contrast, the denitrifiers 'can utilise both RBCOD and slowly biodegradable COD (SBCOD) and the endogenous energy released from the sludge mass due to the death and lysis of organisms, although the respective denitrification rates decrease. Barnard (1988 ) and Ekama et al. (1984) re- ported that 6 and 8.6 mg COD, respectively, were required to reduce 1 mg NO3-N. For complete N removal to minimise the adverse effects of nitrate on BPR, this gives a minimum total COD to total Kjeldahl N (TCOD: TKN) ratio of 6-8.6. Ekama et al. (1984) also found that the minimum RBCOD concentration to simulate anaerobic P release is about 25-50 mg L-1. The TCOD:TKN ratio is a function of the sewage composition and cannot be controlled without external addition of an organic substrate. However, the RBCOD concentration can be increased via primary sludge prefermentation (Barnard, 1988 ).

The available BNR processes can be divided into continuously and inter- mittently operated systems. The intermittently operated systems can be fur- ther subdivided into variable and constant volume systems. The continuous BNR process configuration consists of in-series anaerobic, anoxic and aerobic reactors with an underflow recycle from the settling tank to either the anaer- obic or the anoxic reactor. Nitrification and denitrification occur separately. The nitrate produced in the aerobic reactor is recycled to the anoxic reactor for denitrification. These systems differ principally in the ways of anoxic pre- treatment of the returned activated sludge stream to prevent any nitrate being recycled to the anaerobic reactor (Pitman, 1991 ). The choice of sludge pre- treatment methods and thus the choice of system configurations depends on effluent standards, the influent RBCOD concentration and TCOD: TKN ra- tio and hence the resulting effluent nitrate concentration. When treating was- tewater with unfavourable characteristics, the employment of either the mod- ified University of Cape Town system (Ekama et al., 1984) or the modified four-stage Bardenpho (M4-SB) system (Nicholls et al., 1987) is required. However, the latter is more favourable due to the relatively simpler configu- ration, especially for smaller applications (Ho et al., 1992). In the M4-SB process, a second anoxic reactor is added to the underflow recycle for com-

NUTRIENT REMOVAL IN ACTIVATED SLUDGE SYSTEMS 151

plete nitrate removal before the sludge is recycled back to the anaerobic re- actor. Sludge bulking control in continuous systems is by creating plug flow conditions using baffles or more tanks.

In contrast, the single-tank variable volume intermittent cyclic systems cre- ate reaction conditions to achieve biological N and P removal with sludge bulking control via their inherently flexible repeated time-oriented operating strategy combined with mix or non-mix, aerated or nonaerated, with or with- out feed periods (Ho et al., 1992 ). A typical operating strategy is divided into five discrete time periods: FILL, REACT, SETTLE, DRAW and IDLE. Restricted by the single-tank configuration, i.e. no nitrate recycle, biological N removal in these systems heavily relies on simultaneous nitrification/denitrification during the air-on sequences, eliminating the anoxic conditions required for denitrification in continuous systems. A substantial anoxic mass fraction can exist in the biological floc during the air-on sequences. The magnitude of the anoxic fraction depends on the high initial substrate to biomass (So/Xo) ratio (Chudoba et al., 1992 ), the bulk DO concentration, biomass specific oxygen utilisation rate (OUR) and floc diameter. All these essential reaction condi- tions can be easily achieved simply by incorporating non-mixing sequences which allow the mixed liquor suspended solids (MLSS) concentration to be concentrated at the bottom of the reactor (Ho et al., 1993b). Any residual DO and/or nitrate can be quickly eliminated and incoming waste can be intimate contact with the sludge blanket. Such an operation is known to con- trol sludge bulking (Ho et al., 1993a,b) and is not available in the conven- tional continuous clarifier activated sludge systems.

Consequently, the objective of this paper is to compare the performance of P and N removal and sludge settleability of the modified 4-stage Bardenpho (M4-SB) process and the variable volume intermittently fed and decanted system incorporating two nonmixing sequences (IFID-2NS) on the basis of a laboratory study.

M A T E R I A L S A N D M E T H O D S

Both the bench-scale M4-SB and IFID-2NS processes were seeded with a freshly collected sample of mixed liquor from the Wacol Sewage Treatment Plant in Brisbane. The variable volume IFID-2NS system, which had been operated for 60 days prior to the commissioning of the bench-scale M4-SB process, already showed biological P removal. The respective volumes of the two processes were 15.5 and 4.5 L. They were run in parallel under similar conditions using 18 h hydraulic retention time and 20 days sludge age in a temperature controlled room maintained between 15-20 ° C. The duration of the experiment was 53 days. The physical aspects of the processes and the operating strategy of the intermittent process are shown in Figs. 1 and 2 and Table 1, respectively.

152 K.-m. HO ET AL.

Influent

Aerobic Reactor 3 1 st Anoxic Reactor

Nitrate Anaerobic Reactor ~ recycle AlL Secondary Clarifier

2nd I [ . . Aerobic Reactor 1 Aerobic I Anoxic ( ") Reactor 2 I React°r "~-~

Return AetivateA Sludge

Effluent

Fig. 1. Bench-scale M4-SB process.

5to6L Working Volum©

oid

RAW WASTE WATER

~d

/vlagn©ti¢ Slirrcr

Fig. 2. Laboratory-scale IFID-2NS system.

Grab samples of Wacol raw wastewater were collected once every 5 to 7 days and were stored at 4 ° C. Suspended solids in the wastewater were sepa- rated by gravity sedimentation and transferred to a 5-L batch prefermentor for VFA production, before being mixed with the settled wastewater. The sludge age of the prefermentor was 15-20 days. The feed drums were filled up twice a day. Mixing was provided to keep all solids in suspension. Influent wastewater from the feed drum was fed to the M4-SB system continuously and to the intermittent process intermittently at rates of 750 and 600 mL h - 1,

NUTRIENT REMOVAL IN ACTIVATED SLUDGE SYSTEMS

TABLE 1

Operat ing strategy of the IFID-2NS system

153

React ion sequence Time (h)

1. Nonmixed FILL 1.25 2. Mixed FILL 1.25 3. Aerated mixed REACT 1 1.00 4. Nonmixed REACT 0.50 5. Aerated mixed REACT 2 1.50 6. SETTLE 0.33 7. DRAW 0.17 8. IDLE 0

TOTAL 6.00

respectively, by variable-speed Masterflex pumps. An on-line filter was re- quired to prevent any large solids from blocking the silicon tubing around the pump heads. The filter was cleaned twice a day to prevent solids accumula- tion. Effluent discharge was by gravity. Mixing was provided by either mag- netic stirrer or industrial nitrogen bubbling. Diffuser stones were used to dis- tribute air or nitrogen gas to the reactors. Excess activated sludge wasting was performed manually everyday to maintain the sludge age. All operations (i.e. fill, aeration, mixing, and effluent discharge) were controlled by an IBM com- patible computer.

Daily influent and effluent samples, 12 h and 6 h composite samples re- spectively, were collected. They were then manually composited using an equal volume of daily composite samples to give 5 to 7-day composite samples. Analyses of PO4-P, NO3-N and SO4 on daily composite samples and total suspended solids (TSS), soluble and total COD (SCOD and TCOD ), total P (TP), MLSS, TKN and NH3-N on the manually composite samples were carried out to give the respective nutrient removals and hence the perform- ance of the different systems. In-reactor soluble nutrient concentrations of each batch of wastewater samples were measured to indicate if the treatment was performed satisfactorily after stable operation was noted. Samples for determining soluble parameters were filtered through a 0.45-m membrane. COD concentration was measured by a COD analyser manufactured by HACH. NO3-N, PO4-P and SO4 concentrations were measured by ion chro- matography. Sludge volume index (SVI) was measured using a 250-mL grad- uate measuring cylinder on the basis of a 30-minute settling period. All other analyses were carried out after Standard Methods for the Examination of Water and Wastewater (APHA-AWWA-WPCF, 1985 ).

R E S U L T S

The average influent characteristics during the 53 days experiment period are shown in Table 2. The process performance of the M4-SB and IFID-2NS

154

TABLE 2

Influent and effluent characteristics

K.-m. HO ET AL.

Influent Effluent (average)

Average Day 47 M4-SB IFID-2NS

TKN mgL -1 58.10 57.00 10.91 NH4-N m g L - l 48.57 45.00 9.11 NO3-N mgL -1 0.00 0.00 7.27 TP m g L - l 8.85 10.00 5.22 PO4-P mg L - t 7.43 7.30 4.35 SCOD mgL -~ 153 131 N / A TCOD m g L -1 276 225 21 TSS mg L-I 94 99 N / A TCOD :TKN 4.75 3.90 N / A SCOD: TP 17.27 13.20 N / A

10.07 8.78 1.71 0.83 0.32

N / A 26 N / A N / A N / A

20

PO 4 -P

0 ~ ~ d ' ~ I

0 l0 20 30 40

Day

Fig. 3. Daily effluent nu t r i en t concen t ra t ion - the M4-SB process.

5O

20

I

6O

"7, 15

H ..¢ . lo

g ~ 5 NO3" N

PO -P

10 20 30 40 50

Day

Fig. 4. Daily effluent nu t r i en t concent ra t ion - the IFID-2NS system.

I

6O

NUTRIENT REMOVAL IN ACTIVATED SLUDGE SYSTEMS 15 5

~, 20

~1o e 5

0

<

• NO 3-N [ ] PO 4 .v • NH 4 .N

Fig. 5. Reactor nutrient concentration - the M4-SB process.

25 "7,

e~ 20

~ lo

~ 5 o

0

0 1 2 3 4 5

Time (h)

• NO 3 -N I! PO 4 -P -- NH 4 -N

Fig. 6. Nutrient concentration profile - the IFID-2NS system.

500

400 "7

300

r~

100

1 2 3 4 5 6

Period

Jl 7

• IFID-NS [ ] M4-SB

Fig. 7. SVI of the IFID-2NS and M4-SB processes.

156 K.-m. HO ET AL.

processes, as expressed by the average effluent characteristics and the daily effluent NO3-N and PO4-P concentration, is presented in Table 2 and Figs. 3 and 4, respectively. The biological nutrient removal mechanisms in achiev- ing the final effluent nutrient concentration of each system on Day 47 are illustrated in Figs. 5 and 6. The influent characteristics on Day 47 are also shown in Table 2. The sludge settleability characteristics indicated by the SVI values of the processes are presented in Fig. 7.

DISCUSSION

The average wastewater characteristics of the grab samples collected dif- fered substantially from those of the full-scale plant and were very unfavour- able for BNR. Over a sampling period of more than six months, the average influent wastewater RBCOD, TCOD and TKN concentration, and TCOD: TKN ratio of the full-scale plant were around 30 mg L- 1,400 mg L- 1, 39 mg L- ~ and > 10, respectively. The low influent TCOD concentration could be partially explained by the incorporation of a separate primary sludge pre- fermentor and an on-line filter. Although primary sludge prefermentation gave a high SCOD concentration and a more favourable SCOD: TP ratio, as indi- cated in Table 2, a fraction of influent COD was lost as a result of sulphate reduction and methane production, especially for high influent sulphate con- centration (results not shown) and long sludge age operation (Pitman, 1991 ). Furthermore, reduction of influent TCOD resulting from losses of primary solids during on-line filtration was unavoidable. The separated solids could not be evenly distributed back to the reactors because of the chance of grit accumulation which would eventually lead to the blocking of solenoid valves and silicon tubing. The reduction in TCOD adversely affected the TCOD: TKN ratio.

High average effluent NH3-N and TKN concentrations of both systems were noted (Table 2 ) due to influent TKN concentration and operating tem- perature fluctuations. The TKN concentrations of the grab samples collected in the fourteen 25-L drums varied between 55 and 85 mg L-1. During the winter season, the temperature at night in Brisbane sometimes reached as low as 2 ° C. Limited by the heater capacity, maintaining a constant room temper- ature was impossible. A reactor temperature as low as 11 ° C was recorded. No attempt was made to solve these problems because the objective of the exper- iment was to compare the performance of the two systems operated under similar conditions. Instead, sampling was conducted at different times to compare the results.

Biological N removal in the M4-SB process was poor due to insufficient organic carbon (Table 2, Figs. 3 and 5 ), as predicted by Ekama et al. (1984). Complete denitrification in the first anoxic basin was not possible, as indi- cated by the results of Day 47 (Fig. 5 ). Positive exclusion of NO3-N from the

NUTRIENT REMOVAL IN ACTIVATED SLUDGE SYSTEMS 15 7

stream entering the anaerobic reactor could not be achieved. However, anaer- obic release of 10 mg PO4-P L-~ was recorded resulting in 6.4 mg L-~ en- hanced P removal. It was noted that a fraction of the PO4-P was taken up under the less energy efficient conditions in the anoxic reactor. Consequently, only on one or two occasions was an effluent with a POa-P concentration less than 1 mg L-1 achieved (Fig. 3). Endogenous release of P, as reported by Barnard (1988 ), was not recorded in the second anoxic reactor in the pres- ence of NO3-N. However, endogenous denitrification resulted in digestion of MLSS and an increase in NH3-N concentration.

The IFID-2NS process produced an effluent with much better quality de- spite the low TCOD:TKN ratio (Table 2 and Fig. 4). An effluent POa-P concentration of < 0.5 mg L- t was achieved most of the time. With the gen- eration of a high So/Xo ratio by incorporating a non-mixed FILL sequence at the beginning of a cycle, simultaneous nitrification-denitrification was always recorded with the in-reactor NO3-N concentration being less than < 5 mg L-~ during the entire air-on sequences (Fig. 6 ). It was noteworthy that the organic carbon requirement for biological removal via simultaneous nitrifi- cation-denitrification seemed to be lower than that required when nitrifica- tion and denitrification occurred separately as in the M4-SB process. Conse- quently, nitrogen might be removed in a different pathway either via nitritation-denitritation (NO2-N) resulting in up to 40% saving in organic energy (Abeling and Seyfried, 1992) or via aerobic denitrification (Kugel- man et al., 1991 ). The sludge blanket NO3-N concentration at the end of a cycle was always low which greatly enhanced anaerobic RBCOD uptake and P release. The highest PO4-P in the sludge blanket recorded was 46 mg L -1. POa-P uptake in the presence of DO was rapid and not affected by tempera- ture. Complete elimination of PO4-P was achieved within the first hour of the aeration period with the value remaining low during the rest of the cycle (Fig. 6).

In addition, the IFID-2NS process generated a sludge with good settling behaviour (Fig. 7 ). The low nitrate concentration resulting from simultane- ous nitrification-denitrification enables endogenous anaerobic sludge stabi- lisation. Together with the introduction of a high So/Xo ratio at the beginning of each cycle, synchronised division of microorganisms resulted (Chudoba et al., 1991 ), which further ensured the selection of the floc-forming bacteria. This importance has been demonstrated in other intermittent cyclic processes by Ho et al. (1993b). Consequently, the SVI was lower than 120 mL g- 1 most of the time (Fig. 7 ) despite the high nonaerated mass fraction (approx. 60% ); the result is contrary to that predicted by Ekama et al. (1990) for continuous processes. The increase in SVI during Period 7 was due to a sudden decrease in compressed air flow to the reactor (0.75 instead of 1.25 L min-~ ) coupled with low operating temperatures at night; this reduced the time for biosorp-

158 K.-m. HO ETAL.

tive capacity regeneration and violated the accumulation-regeneration law (Chudoba et al., 1982).

Ekama et al. (1990), however, successfully predicted that the M4-SB sys- tem had an uncontrollable tendency to bulk when a nonaerated mass fraction of 55-60% was used. This was the case in the present system because the pur- pose of adding aerobic reactors 2 and 3 in the lab-scale M4-SB process was to minimise the amount of DO entering the first anoxic reactor and the second- ary clarifier. This was similar to encouraging denitrification in the aerobic basin as reported by Pi tman ( 1991 ) - a recipe for promoting proliferation of filaments. This led to a high SVI at all t ime ranging from 250-420 mL g-1 (Fig. 7). Other explanations were the absence of a high So/Xo ratio in the anaerobic reactor and the presence of anoxic conditions which included an- oxic consumption of the residual substrate entering the first anoxic reactor and endogenous anoxic sludge stabilisation in the second anoxic reactor. The dominant f i lamentous bacteria identified were the low F / M growers - Types 0041/0675 (Jenkins, 1992 ). Although these fi lamentous bacteria were pres- ent in the IFID-2NS reactor, they were mainta ined at moderate number and did not exert any deleterious effects.

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NUTRIENT REMOVAL IN ACTIVATED SLUDGE SYSTEMS 15 9

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