Methodology for Energy Optimization in Wastewater Treatment Plants. Phase I: Control of the Best Operating Conditions
, Francisco del Cerro Velázquez 2,* and Mercedes Llorens Pascual del Riquelme 3,*Round 1
Reviewer 1 Report
The methodology proposed is for energy cost optimization in wastewater treatment plants.
In my opinion, it should be strongly reviewed. The main critical points that I found in the current version of this paper are:
· the cost evaluation study is not experimental, The study evaluates only data about electricity consumption in a year but population served - Waste flow rate and organic load - Type of system - Year of construction of the plants are not expressed
· In the state of art there is generic data about the consumption of electrical energy but it’s referred to different contests: The variables as influent characteristics, plant localization, effluent quality
requirements that affect the selection of the plat configuration and equipment are not considered.
· In the preliminary study, the figures are not related to the text
· There is not a relationship between the concentration of solids and energy consumption;
· It’s not explained how the temperature is controlled
· the influence of temperature on the results is not evaluated
· Characteristic parameters on removal efficiency like the concentration of COD, BOD5 SST..etc. are not measured before and after the treatment
which are the characteristics of wastewater treatment plants considered?
the conclusions with the introduction of automatic control loops are obvious.
Author Response
In response to the proposed comments and suggestions, I would like to say that we have thoroughly revised the proposed methodology for optimizing energy costs in wastewater treatment plants, in order to try to clarify better what we intended at each stage. We attach the version of the revised article, marking in yellow the parts modified according to your suggestions, to facilitate the revision. Although we have also sent it as final to the reviewer.
Responding to the main critical points found in the current version of this document, we comment:
1. We have considered your idea of not including the general data of the installation as a starting point in the experimental section, such as the flow of wastewater, organic load, electricity consumption, population served, type of system, ... That is why we have moved them to the state-of-the-art section. And we have incorporated the year of construction of the plant that were not specified.
However, we consider that our first experimental step to achieve energy optimization was to monitor the specific consumptions of each of the process stages in the WWTP and, after detecting that the biological treatment stage represents the highest percentage of consumption, the components that form it are detailed in order to detect critical points that could be improved.
2. The objective we pursue by including in the state of the art section the energy ratio data, expressed as kWh/m3, in installations with different locations, is to give a global image of the range in which the average energy consumption of a purification installation moves, and to justify the need to establish an optimization methodology when we observe values that are far above the normal average data. The objective is not to compare facilities (technology, wastewater quality, etc.), but to generate in the operator the obligation to evaluate whether the management of the process is optimal or can be improved. Wanting to make a comparison of the relationships between facilities is not a common practice, nor is it simple, given the variables that come into play.
On the other hand, although an attempt has been made, it has been difficult to find data on energy ratios of other facilities similar to the San Pedro del Pinatar WWTP given the peculiarity of their treatment. Therefore, we understand that the use of the data for the desired purpose is valid.
3. The description of all the figures included in section 2.2. with the denomination “Starting energy conditions at the San Pedro del Pinatar WWTP” have been included in the text.
These figures were previously located in the preliminary study section (3.1.) but estimating their suggestion we have changed the location.
4. The relationship between solids concentration and energy consumption is described in section 3.3. between rows 373 and 379.
A higher concentration of solids in the liquor mixture of the biological reactor results in a higher number of microorganisms and therefore in a higher oxygen demand for the degradation of organic matter and for the endogenous breathing process of the bacteria, which results in higher energy consumption for the operation of the aeration blowers.
5. Temperature is an operating parameter that is measured daily in the WWTP, through the dual conductivity and temperature measurement sensor installed at the plant entrance, as a preventive measure against any anomaly that could appear in the input current, due to the presence of uncontrolled spills....., and in the biological reactor the temperature is also measured, through the dual oxygen concentration sensor and the temperature measurement already installed, in order to check the biological activity in terms of degradation of organic matter, oxygen transfer in the reactor ...
In the study we included the new control signals, and we ignored the signals we already had........But estimating their suggestion we have included in section 3.3, in "Automatic sludge purge control system according to the age of the sludge", line 463-464, the way in which we obtain the temperature measurement to obtain the correlation between the variables.
6. Yes, it is evaluated. The influence of temperature on process control parameters (suspended solids in the mixed liquor, oxygen requirements by microorganisms, age of the sludge, etc.) is evaluated in figures 22, 23, 24 and 25, discussed in lines 582-594.
7. The characteristic parameters of the input water COD, BOD5 TSS...etc. have been included in section 2.2.
The characteristics of the wastewater after treatment have not been included, since the general objective of our optimization study is based not only on the reduction of the energy consumption of the WWTP, but also on the maintenance of the quality of the treated water within the values established in the applicable legislation and in the safety of the operation. However, it has been clarified in the new text, in paragraph 3.1, lines 261-265.
Reviewer 2 Report
Comments and suggestions for authors can be found in the attached file.
Comments for author File: 
Comments.pdf
Author Response
In response to the proposed comments and suggestions, I would like to say that we have thoroughly revised the proposed methodology for optimizing energy costs in wastewater treatment plants, in order to try to clarify better what we intended at each stage. We attach the version of the revised article, marking in yellow the parts modified according to your suggestions, to facilitate the revision. Although we have also sent it as final to the reviewer.
Responding to the main critical points found in the current version of this document, we comment:
Review
1. Yes, if data are available for the following years on energy consumption in the installation analyzed, but as discussed in section 3.3, they will be used in the next optimization phases carried out.
2. The objective that we pursue when including in the state of the art section the energy efficiency data in installations with different locations, is to give a global image of the range in which the average energy consumption of a purification installation moves, and to justify the need to establish a methodology for optimization when we observe values that are outside the norm. The objective is not to compare facilities (technology, wastewater quality, etc.), but to generate in the operator the obligation to evaluate whether the management of the process is optimal or can be improved. Wanting to make a comparison of the relationships between facilities is not a common practice, nor is it simple, given the variables that come into play.
On the other hand, it is almost impossible to compare the energy consumption ratios of the San Pedro del Pinatar WWTP with other facilities, given the peculiarity of their treatment. Therefore, we understand that the use of the data for the desired purpose is correct.
Material and methods
3. The characteristic input parameters of wastewater COD, BOD5, TSS..., as suggested to us, have been included in section 2.2.
Total nitrogen, total phosphorus, COD and TSS loads have been included in the graph together with BOD5 data. As you indicate these indicators can affect the operational parameters of the biological treatment process and energy consumption.
4. In paragraph 3.1, lines 261-265, in the new text, this clarification has been included. I would like to point out that all the measures we use to optimize plant consumption are compatible with the WWTP's obligation to maintain the water output parameters in accordance with applicable legislation and to ensure safe operation.
5. The new San Pedro del Pinatar Wastewater Treatment Plant, in Murcia, began to be built in 2005 to replace the old WWTP existing in this municipality, built in the 1990s and whose treatment capacity was insufficient, especially during the summer season when the maximum population is recorded, and also had significant operating deficiencies. The construction of the San Pedro del Pinatar wastewater treatment plant increased its treatment capacity by twofold, with the aim of treating wastewater flows from new housing developments, greater influx of holidaymakers, …. forecasts that were not fulfilled.
A real estate crisis began in 2008, which spread throughout the period studied, and the installation was oversized.
As suggested to us, all these data have been included in the text, having been included in section 2.2. in line 218-226 of the revised article
6. The increase in the inflow of wastewater and pollutant load cannot be predicted.
Yes, it is much better to build the facilities in a modular way, so that they can adapt to the real operating conditions, which can also be variable from one season of the year to another.
This is addressed in the following optimization phases that make up the proposed methodological optimization.
Results
7. The percentage distribution of energy consumption in operations and processes presented in Figures 18-21 are results of the treatment plant analyzed, WWTP of San Pedro del Pinatar, it is not a general data based on the literature.
8. The proposed optimization methodology, with all its phases, can be applied to any treatment process: identification of consumptions by stages, detection of critical points or susceptible to improvement, selection of the optimal operation variables, adjustment of equipment and implementation of a control system.
9. At the San Pedro del Pinatar WWTP the cost of sludge treatment is not very significant, as can be seen in the circular diagrams. As we commented in the article, in wastewater treatment systems with membranes the concentrations of solids are higher, and the sludge is stabilized at the end of the biological treatment. And the sludge treatment line is very simple, limited to a sludge homogenization deposit prior to its passage through the centrifugal dewatering system. It does not require sludge thickening prior to dewatering.
10. The limitation in the operation of the installation working with sludge ages of 3 days, does not come from the need for sludge stabilization, but because the ultrafiltration membranes of the MBR system have a limitation in the concentration of working solids due to premature soiling of the membranes. It is indicated in the article, but as I mentioned in the previous section, the water treatment system by MBR system provides a stabilized sludge without the need to use energy to stabilize the sludge.
11. The part of the system dedicated to the treatment of sludge has been added in the scheme of figure 5. The fate of the dewatered sludge in this WWTP is an agricultural application.
12. The automation and motorization of parameters can be done in different ways, from the most sophisticated to the simplest.
The way in which we evaluate whether the proposed system is recommended or not, is not by an installation size but by evaluating its profitability and amortization period.
Most depuration facilities have fixed-term exploitation contracts, and investments must be made by the operating company. So, in that case, it is sought that the term of amortization of the investment is less than the duration of the operating contract of the installation, in order to recover the investment.
If the repayment term of the investment exceeds the period of operation of the facility, we modify the proposal, or it is rejected.
13. Thank you for your comments, in the new document all these errors have been corrected.
- Fig. 4 - Bad description of the vertical axis - it must be kWh / m3
- Error in the numbering of planes. Twice it is figure 9, not figure 8.
- Figures 24, 25, LMSS - SSLM.
- Schemes in Figures (8) and 16 are distinct.
Round 2
Reviewer 1 Report
The article is sufficiently improved
Reviewer 2 Report
Figure 28: Phase I - Fase I (?)
