Comparison of technology, costs and environmental benefit of wastewater treatment plants in mountainous areas in the alps

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Oxygen supply

Oxygen demand
Oxygen transfer
Conclusions  

Following 4 case studies demonstrate the oxygen demand and –supply of activated sludge systems (Biocos-strategy) within the Life-project:

Oxygen demand

The calculation of the oxygen demand of wastewater loaded activated sludge systems has to consider 3 main components: Demand for degradation of organic compounds: A major portion of organic substances in the (mostly pre-clarified) wastewater influent flow is quickly microbially metabo-lised. About one third is respirated to CO2 in order to produce energy and about two thirds is transformed to body mass of microbial organisms. A part of the organic load is adsorbed and transformed with delay. Endogenous respiration: Independent from the current wastewater loading micro organisms show a basic respiration. Additionally continuous decay of organisms and the metabolism of lyse products requires a minimum amount of oxygen (sludge stabili-sation or -reduction at low loading).  Demand for nitrogen elimination: Oxygen is required for the oxidation of ionised ammonia NH4 to nitrate NO3. A major portion of this amount of oxygen is regained during denitrification (reduction of nitrate to nitrogen N2). A significant portion of the ammonia nitrogen is incorporated into the biomass without any oxygen consumption. O2,demand  = 0.5 * (organ. loading) + 0.1 * (biomass) + 1.71 * (eliminated nitrogen)   The organic loading is calculated in kg BOD per day and the biomass is the product of the sludge concentration multiplied with the volume of the aerated activated sludge tank. The nitrified/denitrified nitrogen load is about half of the influent nitrogen load fed to the biological treatment (10 g N/PE * PEmax / 2). In order to consider daily load variations the maximum oxygen demand per hour is increased by 50 %. A simplified approach assumes an oxygen demand which is about twice as high as the BOD-load.

Berlin Refuge:
O2,demand	= 0.5 * 10.7 kg BOD/d + 0.1 * (9.6m3 * 4.0 kg SS/m 3) + 1.71 * (2.6 kg N/d /2) = 5.35 kg + 3.84 kg + 2.22 kg = 11.4 kg O2/d
O2,demand	= 11.4 * 1.5 / 24 = 0.71 kg O 2 /h
Approach: O2,demand   = 2 * 10.7 kg BOD/d / 24 h = 0.89 kg O2 /h

Coburg Refuge:
O2,demand	= 0.5 * 7.2 kg BOD/d + 0.1 * (10.35 m3 * 3.6 kg SS/m 3) + 1.71 * (1.8 kg N/d /2) = 3.6 kg + 3.73 kg + 1.54 kg = 8.9 kg O2/d
O2,demand	= 8.9 * 1.5 / 24 = 0.55 kg O 2 /h
Approach : O2,Bedarf  = 2 * 7.2 kg BOD/d / 24 h = 0.60 kg O2/h

Konstanz Refuge:
O2,demand	= 0.5 * 4.4 kg BOD/d + 0.1 * (7.2 m3 * 4.0 kg SS/m 3) + 1.71 * (1.1 kg N/d /2) = 2.2 kg + 2.88 kg + 0.94 kg = 6.0 kg O2/d
O2,demand	= 6.0 * 1.5 / 24 = 0.38 kg O 2 /h
Approach : O2,Bedarf  = 2 * 4.4 kg BOD/d / 24 h = 0.37 kg O2/h

Lamsenjoch Refuge:
O2,demand	= 0.5 * 8.0 kg BOD/d + 0.1 * (11.8 m3 * 3.0 kg SS/m 3) + 1.71 * (2.0 kg N/d /2) = 4.0 kg + 3.54 kg + 1.71 kg = 9.25 kg O2/d
O2,demand	= 9.25 * 1.5 / 24 = 0.58 kg O 2 /h
Approach : O2,Bedarf  = 2 * 8.0 kg BOD/d / 24 h = 0.67 kg O2/h

Oxygen transfer

The most efficient oxygen transfer is achieved by compressed air and fine bubble mem-brane aeration. The oxygen input is mainly determined by the air-flow provided by the compressor, which is usually specified by the producer in m3/h. Under standard conditions at 1013 mbar and 20 °C dry air shows an oxygen concentration of 279 g O2/m3. The actual oxygen concentration significantly depends on the altitude:

	... barometric altitude formula
	p(h)... pressure in considered altitude [N/m2] p(0)... pressure at sea level [N/m2 ] r(0)... density of air at sea level [kg/m3 ] g... gravity [N/kg]

The oxygen transfer from compressed air to the liquid shows a specific efficiency h depending on e.g. the size of the bubbles and therefore on the aeration system. Additionally the efficiency of the oxygen transfer depends on the path-length of the rising bubbles and thus on the water depth. Moreover the water depth increases the backpressure the compressor has to overpower. Following approach considers mean efficiency profiles of applied compressors and aerators:

	... efficiency of the oxygen transfer [-]
	H... water depth

The efficiency of the oxygen transfer in wastewater is lower than in pure water which is expressed by the coefficient a (mean empirical value a = 0.8).

Berlin Refuge :
2 * LP200 linear pumps for aeration and 1 * LP200 for syphons
O2,transfer	= 27 m3 /h * η * α * p(2044m)/p(0) * 0.28 kg O2/m3 = = 27 * 0.104 * 0.8 * 0.77 * 0.28 = 0.48 kg O2/h

and

Coburg Refuge :
2 * piston compressors (4.8 und 4.10)
O2,transfer	= 15 m3/h * η * α * p(1917m)/p(0) * 0.28 kg O2/m3 = = 15 * 0.129 * 0.8 * 0.79 * 0.28 = 0.34 kg O2/h

and

Konstanz Refuge :
2 * LP200 linear pump with 12 m3/h each
O2,transfer	= 24 m3/h * η * α * p(1688m)/p(0) * 0.28 kg O2/m3 = = 24 * 0.099 * 0.8 * 0.81 * 0.28 = 0.43 kg O2/h

and

Lamsenjoch Refuge :
2 * piston compressors (4.10 und teilweise 4.8)
O2,transfer	= 13 m3/h * η * α * p(1958m)/p(0) * 0.28 kg O2/m3 = = 13 * 0.129 * 0.8 * 0.78 * 0.28 = 0.29 kg O2/h

and

	Fig. 1:
	Oxygen concentration profiles during a Biocos-operation cycle. The oxygen concentration increases during continuous aeration in the B-tank and decreases during the discharge phase and the recycle phase, when all or a part of the compressed air is used to operate the syphons. Note the different load situations of the treatment plants  (Tab.1).

Fig.1 shows oxygen concentration profiles at single monitoring days. Tab.1 presents the loading conditions at these days: The current oxygen demand is calculated according to the measured sludge concentration, the nitrogen elimination and the organic loading. The comparison indicates that at the WWTP Konstanz Refuge the oxygen transfer is 4 times higher than the demand leading almost to oxygen saturation (6.8 mg O2/l). At the WWTP Berlin Refuge the calculated oxygen transfer is about 60 % higher than the demand and the oxygen concentration exceeds hardly 1 mg O2/l. At the WWTP Lamsenjoch Refuge the calculated oxygen transfer is about 13 % higher than the demand. The measured high oxygen concentration of more than 5 mg O2/l indicates a strong daily variation of the oxygen demand (measurement was done during the relatively low loaded morning period).

site	size [PEmax]	loading [%]	N-eliminated [%]	O2 transfer [kg O2/h]	O2 demand [kg O2/h]	transfer / demand [%]
Berlin	260	40	50	0.48	0.30	160
Coburg	180	17	61	0.34	0.18	189
Konstanz	110	28	65	0.43	0.11	391
Lamsenjoch	200	36	20	0.29	0.26	113

Tab. 1: Calculated oxygen demand (at monitoring dates presented in Fig.1) in relation to the actual oxygen transfer.

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