Solar drying of wastewater sludge: a case study in Marrakesh, Morocco

Pulp-paper industry sludge waste biorefinery for sustainable energy and value-added products development: A systematic valorization towards waste management

The pulp-paper industry is one of the main industrial sectors that produce massive amounts of residual sludge, constituting an enormous environmental burden for the industries. Traditional sludge management practices, such as landfilling and incineration, are restricted due to mounting environmental pressures, complex regulatory frameworks, land availability, high costs, and public opinion. Valorization of pulp-paper industry sludge (PPS) to produce high-value products is a promising substitute for traditional sludge management practices, promoting their reuse and recycling. Valorization of PPIS for biorefinery beneficiation includes biomethane, biohydrogen, bioethanol, biobutanol, and biodiesel production for renewable energy generation. Additionally, the various thermo-chemical technologies can be utilized to synthesize bio-oil, hydrochar, biochar, adsorbent, and activated carbon, signifying potential for value-added generation. Moreover, PPIS can be recycled as a byproduct by incorporating it into nanocomposites, cardboard, and construction materials development. This paper aims to deliver a comprehensive overview of PPIS management approaches and thermo-chemical technologies utilized for the development of platform chemicals in industry. Substitute uses of PPIS, such as making building materials, developing supercapacitors, and making cardboard, are also discussed. In addition, this article deeply discusses recent developments in biotechnologies for valorizing PPIS to yield an array of valuable products, such as biofuels, lactic acids, cellulose, nanocellulose, and so on. This review serves as a roadmap for future research endeavors in the effective handling of PPIS.

Environmental and ecological risk assessment of municipal sewage sludge management using a sustainable solar drying system

Providing a cost-effective and suitable way for sewage sludge refinement with the purpose of reducing environmental impacts or reutilization of sewage sludge can be an important issue for researchers. This study is inclined at bringing an economical and sustainable solution to sludge management in a municipal wastewater treatment plant. Three types of sludges were collected for the experiments: raw sludge was collected during winter (R.S.), digested sludge was collected during winter (D.S.1), and digested sludge was collected during summer time (D.S.2). This study proposes replacing mechanical drying by a solar drying system. Experimental analysis was carried out to determine the total solids (TS), volatile solids (VS), heavy metals concentration, and pollution index (index of geo accumulation, contamination factor, ecological risk factor). The solar drying system was demonstrated to be very efficient with the three samples achieving a dry solid content of 96.96% for R.S., 96.75% for D.S.1, and 93.60% for D.S.2. after solar drying. While pollution index calculations showed that all three sludges present a low potential ecological risk.

Assessing the suitability of solar dryers applied to wastewater plants: A review

Sludge dewatering and drying are the main processes related to sludge management in wastewater treatment plants (WWTPs). Sludge disposal is a high-cost activity, and drying the sludge reduces its mass and volume, resulting in savings in storage, handling and transportation. The discoveries regarding the use of solar energy in agricultural studies provided valuable information for using in sewage sludge drying. Some studies have reported that dry sludge has met the EPA Class A requirement for biosolids using only solar energy as an energy source. The proper sludge mixture, manual or mechanical, and the dewatering process can significantly increase the drying rate, reducing drying time and the surface area needed. The environmental conditions and the sludge's type greatly influence the drying system. A solar dryer system may be suitable to dry different types of wastewater sludge. Modeling techniques can predict the behavior of the solar drying system and, thus, save time and money in experimental steps. CFD modeling of the sludge drying system is usually done by adopting specific boundary conditions and solving the Navier Stokes equations for air and sludge. There is no standard methodology for comparing solar dryers and common methodologies, such as system efficiency and thermal efficiency, disregards different dryers in different operational conditions. A SWOT (strengths, weaknesses, opportunities, and threats) analysis indicated that, in general, the chapel-type greenhouse with mixed-mode drying has higher drying rates, resulting in reduced drying time and can be scaled to any size. Thus, this type of dryer emerges as a more economical alternative to commercial solar dryers. Based on a systematic review, this work points the SWOT analysis as a useful tool for selecting solar dryers.

Thermal and mass transfer properties of a shrinkable industrial sludge: experimental determination and modeling approach

In this study the thermo-physical and mechanical properties of an industrial sludge like shrinkage coefficient, density, water activity, apparent moisture diffusivity, effective thermal conductivity and heat capacity are determined experimentally. These properties constitute a major part of the required input data to model the sludge drying. The sludge shrinks linearly until reaching a critical water content X_cr = 23% and then maintains a constant volume until the end of drying. Therefore, the density increases with decreasing the water content until the critical value (X_cr = 23%) then it decreases until the end of drying. The Henderson equation seems to be the most appropriate statistical model to describe the water sorption isotherm of the sludge. The apparent moisture diffusivity is determined from the numerical solution of Richard's law of diffusion with shrinkage. The results show that the effective thermal conductivity and the heat capacity decrease with the reduction of the water content from 2.60 W m^-1 K^-1 and 2.52 J g^-1 K^-1 for a wet sludge to 1.71 W m^-1 K^-1 and 0.78 J g^-1 K^-1 for a dry sludge respectively. The relationships between thermo-physical / mechanical properties and the water content / temperature of the sludge are established to be introduced in the created numerical drying model.

Occurrence and removal of intestinal parasites in two wastewater treatment plants in the south of Morocco

BACKGROUND

Agadir city plans to reuse treated wastewater on a large scale to irrigate these green spaces. This study aims to evaluate the risk of intestinal parasites, to which human and animal populations are exposed during the reuse of treated wastewaters.

METHODS

Over 21 months, wastewater samples were collected in Bensergao and Mzar wastewater treatment plants, in Agadir city, southern Morocco. The occurrences and removal of intestinal parasites were inspected using the modified Bailenger method.

RESULTS

The biodiversity of helminths is higher than protozoa, while the protozoa cysts are more abundant and more prevalent. Generally, the highest parasitic loads were recorded during the hot season. For raw waters, the average concentration of protozoa cysts was 173.33 ± 70.81 cysts per L for Bensergao plant and 179.33 ± 129.22 cysts per L for Mzar plant. However, the helminths average concentration was 48.70 ± 39.91 eggs/larvae per L for Bensergao plant and 51.10 ± 31.76 eggs/larvae per L for Mzar plant. The purified water of the Mzar plant does not contain parasites, unlike the Bensergao plant where a small numbers of parasites were detected, included 2.33 ± 1.53 cysts per L for protozoa and 0.45 ± 0.58 eggs/larvae per L for helminths.

CONCLUSIONS

Considering parasitological risks, the purified waters of Bensergao plant cannot be used without risks to the public health and environment. In contrast, the purified waters and the purified waters disinfected by ultraviolet radiation from the Mzar plant can be used safely to water the green spaces of Agadir city.

Industrial symbiosis of anaerobic digestion and pyrolysis: Performances and agricultural interest of coupling biochar and liquid digestate

The sustainability of the anaerobic digestion industry is closely related to proper digestate disposal. In this study, an innovative cascading biorefinery concept coupling anaerobic digestion and subsequent pyrolysis of the digestate was investigated with the aim of enhancing the energy recovery and improving the fertilizers from organic wastes. Continuous anaerobic co-digestion of quinoa residues with wastewater sludge (45/55% VS) exhibited good stability and a methane production of 219 NL CH₄/kg VS. Subsequent pyrolysis of the solid digestate was carried out (at 500 °C, 1 h, and 10 °C/min), resulting in a products distribution of 40 wt% biochar, 36 wt% bio-oil, and 24 wt% syngas. The organic phase (OP) of bio-oil and syngas exhibited higher and lower heating values of 34 MJ/kg and 11.8 MJ/Nm³, respectively. The potential synergy of coupling biochar with liquid digestate (LD) for agronomic purposes was investigated. Interestingly, coupling LD (at 170 kg N/ha) with biochar (at 25 tons/ha) improved the growth of tomato plants up to 25% compared to LD application alone. In parallel, co-application of biochar with LD significantly increased the ammonia volatilization (by 64%) compared to LD application alone, although their simultaneous use did not impact the C and N mineralization rates.

Sustainable management of biological solids in small treatment plants: overview of strategies and reuse options for a solar drying facility in Poland

The issue of sustainable management of biosolids (excess sludge) from wastewater treatment is an important issue in the entire developed world. Residual sludge disposal costs and environmental impact may be significant, and reducing such costs, as well as the energy consumption for dewatering and drying, is a key issue for safe and sustainable sludge disposal, considering the recent ban of some disposal options, such as landfilling, in many European countries. An alternative to thermal technologies is solar drying (not to be confused with bio-drying, very close to the concept of composting). Solar greenhouse drying technology is characterized by reduced land requirements compared with traditional outdoor drying beds, as well as by low-energy requirements compared with other thermal drying methods. Process operation is cost-efficient, with close to no maintenance, and observed specific evaporation rates up to threefold higher than conventional drying beds. Many applications of this technology exist in Poland, Germany and Austria: more than 10,000 t of wet sludge per year is treated in this way in Germany alone and almost as many (9000 t/year) in Poland. This paper examines current biosolids treatment technologies applicable to small wastewater treatment plants (2000-9999 population equivalents served) and opportunities for possible solids reuse in Poland in view of sustainable circular economy schemes. In particular, a purely solar-driven greenhouse facility for sewage sludge drying was investigated under different conditions (season, temperature, environmental humidity) and possible improvements for its efficiency evaluated. Sludge processed by solar drying could have different final disposal pathways, according to season, in accordance with the prescriptions of the new National Waste Management Plan of Poland.

Evaporation and concentration of desulfurization wastewater with waste heat from coal-fired power plants

The reduction of wet desulfurization wastewater is one of the important tasks of coal-fired power plants, and it is important for achieving "zero emissions." Evaporation and concentration (E&C) with waste heat is an effective way to reduce wastewater. Here, two typical types of industrial desulfurization wastewater are used to study the change rule of pH and total dissolved solids during wastewater concentration in a circulating evaporation tower. The results indicate that with the increase of concentration ratio, the pH of desulfurization wastewater is decreased rapidly and then is gradually stabilized at 2-3 when SO₂ or SO₃ is contained in flue gas, and the increase in conductivity is less for wastewater with higher SO₄^2- content. The characteristics of various ions are also analyzed, and the composition and microscopic morphology of the precipitates are characterized during concentration. The growth pattern of Ca²⁺ concentration is dependent on the ratio of Ca²⁺ and SO₄^2- in raw wastewater. When the concentration ratio is 7.21, the insoluble and slightly soluble substances undergo precipitation and the solid content is approximately 20%, which can help realize the concentration and reduction of desulfurization wastewater.