The selected aerator for aquaculture operations must be economically efficient and should be able to fulfil the requirement of oxygen supply in the pond water. Here we present a study of economic feasibility of nine different types of aerators using life cycle costing (LCC) approach. The recommended option can be implemented for any types of cultured species at any prevailing environmental conditions.
The aquaculture sector plays a vital role in meeting food and nutritional demand worldwide. To meet the growing demand, fish farmers are increasing production by adopting semi intensive and intensive aquaculture system.
At high stocking density, dissolved oxygen (DO) is probably the second most important input next to feed regulating the production of fish. Therefore, it is essential to supply DO through artificial aeration in these types of culture systems.
“Hence, the continuous supply of oxygen for maintaining the adequate DO concentration to the aquaculture ponds has become prerequisite for healthy growth and survival of aquatic species.”
Aerators can induce circulation of water, supply of DO, remove small or large size particles and improve bottom mud conditions. But, the knowledge of efficient utilization of aerators to contribute to sustainable production systems is poorly understood.
Many farmers operate the aerators without knowing its suitability of use and efficiency. Such empirical practices may not be very beneficial to farmers because the management expense of aerators may be prohibitive.
“Therefore, systematic operation of the aerator with proper knowledge is required from both economic and environmental points of view. It’s essential to create research possibilities to reduce the energy usage of aeration in aquaculture.”
This article present a study conducted to evaluate the economics of different existing aerators. The capitalization method, a LCC approach, was used for comparing the economics among different aerators. In this method, the total capitalized cost is determined by adding the capital cost of the aerator with the capitalized values of replacement cost, maintenance cost, and the energy cost.
A typical Indian major carp (IMC) culture is considered in this study and based on the oxygen demand posed by the IMC, phytoplankton’s, benthos, the capitalized cost of nine different aerators were found out for different conditions (initial DO levels and pond volumes) and the aerator with the least capitalized cost was recommended for each condition.
Materials and methods
Life cycle costing approach
Life cycle costing (LCC) is an economic analysis technique to evaluate the total cost of a system over its life span or over the period a service is provided. In this technique, two costs are considered – (i) capital cost and (ii) recurring cost.
Thus will be the summation of capital cost, replacement cost for the product, capitalized maintenance cost and capitalized energy cost and is represented as follows.
where, C0 is the product capital cost, CR is the capitalized replacement cost, CM is the capitalized maintenance cost, and CE is the capitalized energy cost.
The capitalized cost of various alternatives can be calculated using the above equations and the optimal configuration will be the one with the least capitalized cost. Following the method as described and assuming capital cost includes the cost of required aerators and standby aerators as a factor of safety, the capitalized cost of the aerators was also calculated.
Aeration characteristics of various aerators
Different types of aerators are being used in aquaculture operations. The aerators in general can be classified under three categories:
(i) Splash Aerator,
(ii) Diffused-air Aerator and
(iii) Gravity Aerator.
In the present study, nine different types of available aerators:
(i) perforated pooled circular stepped cascade (PPCSC);
(ii) pooled circular stepped cascade (PCSC);
(iii) circular stepped cascade (CSC);
(iv) paddle wheel (PWA);
(v) spiral aerator (SA);
(vi) propeller aspirator pump (PAA);
(vii) submersible aerator (SUBA),
(viii) impeller aerator (IA) and
(ix) air-jet aerator (AJA) (Figure 1) were considered.
The aeration characteristics of the aerators are reported in Table 1.
Method for evaluation standard oxygen transfer rate (SOTR) and standard aeration efficiency (SAE) of an aeration system
In order to evaluate the performance of an aerator, standard oxygen transfer rate (SOTR) and standard aeration efficiency (SAE) are generally used. These parameters are defined as follows:
The standard oxygen transfers rate (SOTR) of an aeration system is defined as the oxygen transfer per unit time to a water body under standard conditions.
Standard aeration efficiency (SAE) is a better comparative performance parameter than SOTR (Lawson and Merry, 1993) which is defined as the standard oxygen-transfer rate (SOTR) per unit of power input to the aerator.
Estimation of total oxygen demand (TOD)
The total oxygen demand (TOD) in aquacultural pond depends on the cultured species as well as the quality of water. In the present study standard culture of Indian Major Carps (IMC), namely, catla (Catla catla), rohu (Labeo rohita) and mrigal (Cirrhinus mrigala) was considered with a ratio of 4:3:3. The different culture parameters are presented in Table 2.
AE and the number of aerators required directly depend on the water quality conditions which mainly include water temperature, T; α and β factors, initial DO concentration in pond water (CP) and pond water volumes.
In the present study, typical values of temperature (T), α and β were chosen as 25 ºC, 0.95 and 0.90 respectively. The saturation dissolved oxygen concentration, Cs was considered as 8.26 mg/L at 25 ºC. CP (1, 2 and 3 mg/L) as well as the pond water volumes (500, 1000, 2000, 3000, 5000, 8000 and 10,000 m3 with water depth as 1.0 m in each of the cases) were varied to evaluate the life cycle cost for different aerators.
Estimation of total capitalized cost of aeration system (Cc)
The values of capital cost and life span of the different aerators are presented in Table 3. The bank interest rate, maintenance fraction, salvage fraction, and standby fraction for the aerators were assumed to be 12%, 0.07, 0.10, and 0.75 respectively.
The electricity rate was taken as Rs. 4.72/kWh. Assuming the culture period to be of 190 days per year with 16 h of aeration time per day, the total annual hours of aeration was calculated to be 3,040 h.
The capital cost, capitalized replacement cost, capitalized maintenance cost, and capitalized energy cost were calculated assuming the bank interest rate, maintenance fraction, salvage fraction, and standby fraction as mentioned above.
The total capitalized cost of the nine different types of aerators were then calculated using Equation (2) for different values of CP (1, 2, and 3 mg/L) and pond water volumes (500, 1000, 2000, 3000, 5000, 8000, and 10,000 m3 ).
Economic comparison was eventually made among the different aerators based on the total capitalized cost. Finally, the optimal type of aerator was recommended based on the values of CP and pond water volumes.
Capitalized cost of different aerators
The capitalized cost of all the aerators considered at different values of CP and pond water volumes are presented in Table 4.
The aerator with the minimum capitalized cost at a particular CP and pond water volume is denoted in bold. It can be noted from the table that at pond water volumes of less than 2,000 m3 , the minimum capitalized cost (Cc) is achievable with the PPCSC aerator, particularly at low values of CP (CP < 3 mg/L, critical condition in case of an intensive aquacultural pond).
However, at higher pond water volumes (more than 2,000 m3 ) and CP ≥ 3 mg/L, IA is the most preferred aerator. The suitability of various aerators for different CP (mg/L) values and pond water volumes (V) based on the minimum capitalized cost is presented in Table 5.
It can be observed that, PPCSC aerator can be considered as the most suitable aerator (yielding the lowest capitalized cost), for the following combinations:
(i) CP = 1 mg/L and V ≤ 2,100 m3 ,
(ii) CP = 2 mg/L and V ≤ 2,800 m3 and
(iii) CP = 3 mg/L and V ≤ 1,800 m3 .
Under other situations, mostly when pond water volume (V) is more, IA proves to be the most suitable aerator followed by PWA, PPCSC and other available aerators.
It is important to note that in intensive fish culture systems, it has become a common practice to adopt small pond sizes for proper management of the culture system. In such culture ponds, PPCSC aerator will be the most preferred one in comparison with the other available aerators.
The capitalized cost of aerators depends on various parameters like culture species, oxygen demand by cultured species, planktonic and benthic species, stocking density, the environmental conditions of the pond water, including temperature, initial dissolved oxygen (CP), and pond water volumes (V).
Apart from CP and V, two significant parameters affecting the economics of the aerators are stocking density of fish and the capital cost of the aerator as they have a direct impact on the values of Cc for different aerators.
In this analysis, only two best performing aerators – PPCSC and IA have been selected for comparison.
In order to evaluate the effect of stocking density of fish on Cc, a sensitivity analysis was performed for PPCSC and IA by varying the stocking density by ± 20% for different pond water volumes (V) of 500, 1000, 2000, 3000, 5000, 8000, and 10,000 m3 and CP values (1, 2, and 3 mg/L).
It can be concluded that when stocking density is decreased by 20%, PPCSC aerator is preferable with V less than equal to 2,000 m3 at all values of Cp. The results for CP =3 mg/L is shown in Figure 2.
The variation of capitalized cost due to changes in capital cost (± 20%) of the aerators at different pond water volumes for CP values of 1, 2, and 3 mg/L was calculated. The results for CP =3 mg/L is shown in Figure 3.
It can be clearly noticed from the figures that, PPCSC aerator performs better than IA up to V = 2,000 m3.
However, at higher values of V, economic performance of IA is better. This life cycle costing approach for selection of aerators can very well be applied to any types of culture systems. However, the input parameters involving the culture of the specific aquatic species have to be modified, and the capitalized cost for all the aerators can be calculated and compared accordingly.
A framework for comparative economic analysis among aerators for use in aquacultural ponds has been developed based on capitalization method, a life cycling costing approach.
Comparative economic analysis assuming a typical IMC culture reveals that
(i) PPCSC aerator is economically the most efficient aerator when pond water volume is less than equal to 2,100 m3 and the pond dissolved oxygen is critically low (less than equal to 3 mg/L).
(ii) For pond water volumes more than 2,100 m3 , IA is the most suitable aerator followed by PWA, PPCSC and other aerators.
(iii) The sensitivity analysis of the capital cost of the aerator and the stocking density on the capitalized cost of the aerators revealed that up to 2,000 m3 of pond water volume, PPCSC aerator is economically better than IA.
However, for higher pond water volumes, IA performs the best on economical basis.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “ECONOMIC FEASIBILITY STUDY OF AERATORS IN AQUACULTURE USING LIFE CYCLE COSTING (LCC) APPROACH” developed by SUBHA M. ROY, RAJENDRA MACHAVARAM and C.K. MUKHERJEE – Indian Institute of Technology Kharagpur, West Bengal, India; SANJIB MOULICK – School of Civil Engineering, Kalinga Institute of Industrial Technology (KIIT) Deemed to be University, Bhubaneswar, Odisha, India.
The original article was published in JOURNAL OF ENVIRONMENTAL MANAGEMENT in NOVEMBER 2021.
The full version, including tables and figures, can be accessed online through this link: https://doi.org/10.1016/j.jenvman.2021.114037