Criteria for sizing volumes of activated sludge reactors for wastewater from the recycled paper industry: A new approach to projects

doi:10.21203/rs.3.rs-3210587/v1

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Criteria for sizing volumes of activated sludge reactors for wastewater from the recycled paper industry: A new approach to projects

https://doi.org/10.21203/rs.3.rs-3210587/v1

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In response to the need for obtaining sizing parameters for activated sludge reactors used for treating effluents from the recycled paper industry, batch reactors were tested at 20°C under different hydraulic retention times (HRT) and 3 ranges of mixed liquor volatile suspended solids (MLVSS) concentrations. Kinetic equations for the removal of BOD₅ and COD were obtained, along with a model equation for calculating the volume of activated sludge reactors. Removals above 90% for BOD₅ and COD were recorded with 72 hours of aeration and MLVSS concentrations above 4 g L^− 1.

Pollutant removal

reaction volume

pulp and paper

removal kinetics

The paper industry holds significant importance in the global landscape (Liang et al., 2021). This industrial activity generates high volumes of wastewater (WW), containing harmful organic and inorganic pollutants detrimental to the environment (MUNIR et al., 2022). According to Ashrafi et al. (2015), the production of one ton of paper generates 5 to 100 m³ of WW, which globally accounts for 42% of the three billion tons of industrial effluents produced on the planet (Toczylowska-Maminska, 2017).

As a primary treatment, these WWs need to go through stages of separating non-biodegradable solids in flotation tanks (MUNIR et al., 2022). As a secondary treatment, biological systems are recommended (Mendonça et al., 2022), although data for treatment system designs for this type of wastewater are non-existent in the literature. In light of the need to establish a reliable guideline for sizing treatment plants for wastewater derived from the recycled paper industry, this study tested activated sludge reactors in batches for the purpose of removing pollutants and nutrients from this type of wastewater. The study outlined parameters for sizing and proposed an equation to determine the volume of reactors.

Bench-scale tests were conducted in three 1.5 L reactors (R1, R2, and R3) with diffused air aeration (DO ≤ 3.0 mg L^− 1) and reaction times of 12, 24, 32, 48, 60, and 72 hours to express the removals of BOD₅ and COD. The sedimentation time for each test was 30 minutes. Three ranges of mixed liquor volatile suspended solids (MLVSS) concentrations were tested: 3,000–3,500 mg L^− 1 for R1, 4,000–4,500 mg L^− 1 for R2, and 5,000–5,500 mg L^− 1 for R3. Significant differences between treatment means were checked using the Tukey test at a 95% confidence level. Before performing the parametric tests, the normality of the data was confirmed by the Shapiro-Wilk test using PAST software. In each reactor, 250 mL of sludge with an age (θc) of approximately 12 days was inoculated. The inoculum volume corresponded to 1/7 of the reactor. The main bacterial species in the inoculum were Zooglea ramigera, Sphaerotilus natans, Microthrix parvicella, Thiothrix sp, Beggiatoa sp, and Pseudomonas sp, detected through metagenomic analysis (Laudadio et al., 2019). To maintain the DBO:N:P ratio at 100:5:1, 5 mg of nitrogen (N) and 1 mg of phosphorus (P) were added. Urea (40 mg L-1) was used as the nitrogen source, and phosphoric acid (7.8 mg L-1) as the phosphorus source. The pH was maintained between 6.5 and 7.5, and the temperature at 20°C (± 1°C). The three batch tests were repeated 10 times., and samples were analyzed in triplicates. Analytical procedures were performed in accordance with APHA (American Public Health Association) standards from 2017.

BOD₅ Removal and Equation for Reaction Volume Sizing

The BOD₅ removals were above 90% for the 72-hour HRT under the conditions of applying MLVSS concentrations of 4000–4500 and 5000–5500 mg L^− 1, making this the ideal recommendation for the sizing of activated sludge systems (Table 1 and Fig. 1a).

Table 1

Removal of the 6 main operating parameters
Parâmetros	MLVSS 3,000–3,500 mg L^− 1			MLVSS 5,000–5,500 mg L^− 1
BOD₅ (mg L^− 1)
To treat	1100₍₁₀₎		1100₍₁₀₎	1100₍₁₀₎
HRT 24	550₍₂₀₎^a		300₍₅₎^b	292_(1.1)^b
HRT 48	360₍₁₄₎^a		142₍₃₎^b	134_(2.4)^b
HRT 72	229_(6.2)^a		90_(0.5)^b	80_(0.3)^b
COD (mg L^− 1)
To treat	2400₍₆₄₎		2400₍₆₄₎	2400₍₆₄₎
HRT 24	950₍₁₂₎^a		590_(26)b	500₍₇₎^b
HRT 48	605₍₉₎^a		312₍₁₀₎^b	273_(1.1)^b
HRT 72	290_(0.6)^a		110_(2.3)^b	100_(1.9)^b
O&F (mg L^− 1)
To treat	30_(0.5)		30_(0.5)	30_(0.5)
HRT 24	20_(0.3)^a		15_(0.1)^a	10_(0.3)^b
HRT 48	12_(0.1)^a		9_(0.02)^a	4_(0.01)^b
HRT 72	10_(0.0)^a		0_(0.0)^b	0_(0.0)^b
Surfactants (mg L^− 1)
To treat	8_(0.02)	8_(0.02)		8_(0.02)
HRT 24	2_(0.0)^a	1.1_(0.03)a		0.5_(0.0)b
HRT 48	0.5_(0.0)^a	0.6_(0.0)a		0.0_(0.0)b
HRT 72	0.0_(0.0)^a	0.0_(0.0)a		0.0_(0.0)a
N-NTK (mg L^− 1)
To treat	50₍₃₎		50₍₃₎	50₍₃₎
HRT 24	12_(0.2)^a		10_(0.0)^a	8_(0.01)^a
HRT 48	8₍₁₎^a		3_(0.0)^b	1_(0.0)^b
HRT 72	3_(0.02)^a		0_(0.0)^a	0_(0.0)^a
TSP (mg L^− 1)
To treat	20_(0.2)		20_(0.2)	20_(0.2)
HRT 24	18_(0.5)^a		15_(0.02)^a	13₍₁₎^a
HRT 48	15_(0.1)^a		10_(0.0)^b	8_(0.0)^b
HRT 72	10_(0.1)^a		4_(0.0)^b	3_(0.0)^b
O&F – Oils and fats; TPS – Total soluble phosphorus

For the 48-hour HRT and the same MLVSS concentrations, the BOD₅ removals ranged between 85 and 87%, which can also be adopted in situations where discharge standards in watercourses are less stringent. However for the 24-hour HRT, the BOD₅ removals were between 50 and 73%, and thus, not recommended.

In Fig. 1, it can be observed that the decay process of BOD₅ is more intense up to 24 hours, reaching up to 73% removal. However, even with the significant MLVSS concentration used, the 90% efficiency is only achieved after 72 hours of HRT. There was no statistical difference for the 72-hour HRT when applying 4000–4500 and 5000–5500 mg MLVSS L^− 1.

The equation that determines the decay kinetics obtained in Fig. 1 was used in the proposed equation for reactor sizing.

$$\text{V}\text{r}=\frac{\text{Q} \times \text{D}\text{B}\text{O}\text{i}\text{n}-940{\times \text{e}}^{(-0.036 \times \text{H}\text{R}\text{T})}}{1000\times F/M\times \text{M}\text{L}\text{V}\text{S}\text{S}} \left(1\right)$$

Were:

Q (m³ d^− 1) = flow rate; DBOin = Initial BOD₅ concentration (mg L^− 1); HRT = Hydraulic retention time (minutes); F/M = food-to-microorganism ratio; MLVSS (g L^− 1).

The term in the equation: $940 \text{x} {\text{e}}^{(-0.438 \times \text{H}\text{R}\text{T})}$, was obtained from the kinetic curves of the bench tests and corresponds to the final BOD obtained by the treatment system as a function of the HRT (Fig. 1a).

The Eq. 1 can be used under the following conditions:

DBOin (influent to the reactors): ≤ 1,200 mg BOD₅ L^− 1

MLVSS range: 4,000 to 5,500 mg L^− 1 (equivalent to 4 to 5.5 g/L)

F/M (Food to Microorganism ratio): 0.075 to 0.10

HRT (Hydraulic Retention Time): 48 to 72 hours for 87 to 92% BOD₅ removal

Sludge age (θc): 18 to 30 days.

Under these conditions, Eq. 1 can be applied for the sizing of activated sludge reactors.

Example

$\text{V}\text{r}=\frac{100 \times (1000-940{\times \text{e}}^{\left(-0.036 \times 72\right)})}{1000\times 0.08\times 4}=290 \text{m}³$

COD Removal

COD removals above 95% were achieved only for the 72-hour HRT at MLVSS concentrations of 4000–4500 and 5000–5500 mg L^− 1, which is the recommended range for full-scale unit sizing (Table 1). During the same period (72 h) and at concentrations between 3,000–3,500 mg L^− 1, COD removals of 88% were recorded. For COD removal (Table 1 and Fig. 1b), a similar pattern to BOD₅ decay was observed up to 24 hours of HRT. However, after 24 hours, 75% of the applied COD was already removed in the reactor operated with 4000–4500 mg MLVSS L^− 1. Under these conditions, the reactor showed a COD removal of 91% from 60 hours onward. Similar to BOD₅, there was no statistical difference for the 72-hour HRT when applying 4000–4500 and 5000–5500 mg MLVSS L^− 1.

The removal of other parameters

The six main operating parameters of this research are presented in Table 1. Next to each average concentration, there is the standard deviation in parentheses, and superscript letters are used to indicate statistical significance. If two concentrations have the same letter, it indicates that there is no statistically significant difference between them. If the letters are different, it means that there are significant differences. For 72 hours of HRT and MLVSS concentrations above 4000 mg/L, the values of NTK (Total Kjeldahl Nitrogen), O&F (Oil and Fat), and surfactants were below the method's detection limit, and phosphorus (P) was removed with an efficiency of 85%.

BOD₅ and COD were satisfactorily removed with HRT between 48 and 72 hours and MLVSS concentrations of 4000 mg/L. The parameters for the sizing of activated sludge systems should be within the range of 4000–5000 mg MLVSS L^− 1 and F/M between 0.075 to 0.1. The equation obtained from the BOD₅ decay curves can be applied for reactor sizing. The concentrations of NTK, O&F, and surfactants are completely removed at the 72-hour HRT with MLVSS values above 4000 mg/L.

Ethical Approval: Not applicable

Data Availability Databases can be made available on request to the authors.

Consent to Publish All authors grant all publication rights for this work.

Competing Interests The author declare no competing interests.

Consent for Publication The author agree with the last version of the article.

Authors Contributions

Henrique Vieira de Mendonça: Resources, Investigation, Conceptualization, Validation, Writing – original draft, Writing – review & editing.

Funding: This study was funded by Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro – FAPERJ, process number: E-26/210.807/2021.

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Version 1

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Criteria for sizing volumes of activated sludge reactors for wastewater from the recycled paper industry: A new approach to projects

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Results and Discussion

BOD₅ Removal and Equation for Reaction Volume Sizing

COD Removal

The removal of other parameters

Conclusions

Declarations

References

Status:

Version 1

Criteria for sizing volumes of activated sludge reactors for wastewater from the recycled paper industry: A new approach to projects

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Results and Discussion

BOD5 Removal and Equation for Reaction Volume Sizing

COD Removal

The removal of other parameters

Conclusions

Declarations

References

Status:

Version 1

BOD₅ Removal and Equation for Reaction Volume Sizing