The objective of this work is to demonstrate that some weaknesses of the onsite packaged WWTP associated with high operational costs and energy inefficiency could be overcome by improved management. The research methodology consists of series of batch studies with sludge from municipal or onsite WWTP, which simulate different working regimes of the onsite WWTPs – daily operation, toilet flushing and dishwasher machine. A simple classical tool, Oxygen Uptake Rate (OUR) is used to prove the hypothesis that regardless the specificity of the onsite WWTPs, namely the irregularity of the flow and load, three parameters follow similar increasing and decreasing trends – inflow rate, inflow pollution load and oxygen demand in the reactor. The literature review has not shown research publication about applicability of (OUR) for management of onsite WWTPs, but has shown experience and knowledge with municipal WWTPs, which were utilized in our study. The results prove that when there is no wastewater generation in the household, the (OUR) in the reactor is very low, 0.0007 to 0.0015 mg/l.s, thus do not require high oxygen supply. However, when wastewater flushes into the onsite WWTP, the oxygen demand increases rapidly and (OUR) reaches the range of 0.0040 to 0.0063 mg/l.s depending on the type and the quantity of the incoming substrate (pollution load). These results, if verified in filed experiments will enable optimization of the energy use during onsite WWTP operation. The suggestion is that the oxygen supply in the reactor should be adjusted according to the demand, respectively proportional to the inflow rate. In addition to the benefit of saving energy, the comprehensive sensors for dissolved oxygen monitoring, which require qualified maintenance could be avoided and replaced by simple sensors for level, which are anyway part of the equipment of most of the onsite packaged WWTP.

Algae-based wastewater treatment technologies are promising green technologies with huge economical potential and environmental co-benefits. However, despite the immense research, work, and achievement, no publications were found wherein these technologies have been successfully applied in an operational environment for nitrogen and phosphorus removal of secondary treated effluent in municipal wastewater treatment plants. Based on a literature review and targeted comprehensive analysis, the paper seeks to identify the main reasons for this. The reliability (considering inlet wastewater quality variations, operating conditions and process control, algae harvesting method, and produced biomass) as well as the technology readiness level for five types of reactors are discussed. The review shows that the reactors with a higher level of control over the technological parameters are more reliable but algal post-treatment harvesting and additional costs are barriers for their deployment. The least reliable systems continue to be attractive for research due to the non-complex operation and relieved expenditure costs. The rotating biofilm systems are currently undertaking serious development due to their promising features. Among the remaining research gaps and challenges for all the reactor types are the identification of the optimal algal strains, establishment of technological parameters, overcoming seasonal variations in the effluent’s quality, and biomass harvesting.

Background: The management of sludge from municipal wastewater treatment plants (WWTPs) is a global issue, but also an opportunity for circular reuse. Recent data show that sludge reuse in agriculture has the highest share of all utilization routes in the EU. Council Directive 86/278/EEC on the spreading of sludge in agriculture, adopted more than 35 years ago, is still relevant, notwithstanding the discussion on the need to update it. Extracting critical, strategic, and precious metals and metalloids from sludge is an alternative for sludge reuse, which offers several benefits, such as avoiding the high environmental and health risks associated with using sludge directly in agriculture. Additionally, it allows for the recovery of metals, including those listed as Critical Raw Materials by the European Commission. To implement this alternative, it is necessary to first assess the metal content in the sludge and then develop economically and technically viable technologies. In this study, the content of chemical elements in the sludge of eight full-scale WWTPs in Bulgaria is analysed with focus on: (1) assessing the suitability for agricultural application by evaluating the content of macro- and micronutrients and hazardous metals; (2) assessing the possibility of using the sludge as a source of critical and precious metals. Results: For the main nutrients, the following contents as a percentage of the sludge dry weight (DW) were recorded—2.06% to 6% for N, 1.52% to 2.67% for P and 0.47% to 0.81% for K, which are in line with case studies of successful sludge application in agriculture. Only sludge samples from two WWTPs exceeded the permitted limit for hazardous metals and metalloids. On the other hand, of the 21 metal and metalloid constituents listed in the EU Critical and Strategic Material (CRM) list, at least one of the examined samples has a content above 10 mg/kg for 15 elements. The average contents in mg/kgDW of Au (1.1), Al (19,272.9), Mg (6677.6), Ti (1730.9), Ga (20.9) and As (16.6) measured in the investigated WWTPs are among the highest or second highest reported in other countries. Conclusions: The results of the study show prospects for optimising and improving the reuse of sewage sludge in Bulgaria. Sewage sludge from most WWTPs has potential for agricultural application due to its high nutrient content. Large amounts of accumulated critical and strategic metals, gold and silver are trapped in Bulgarian sewage sludge, indicating that sewage sludge could be considered an alternative source with high potential for these valuable elements.

Urban water systems are still in their infancy regarding the transition toward a circular economy, despite the sporadic successful examples worldwide. This paper was aimed at analyzing the preparedness of four water utilities in Bulgaria for the implementation of circular economy principles and solutions. These utilities provide water supply and sewerage services to about 30% of the population in Bulgaria. SWOT analysis was used as a core tool. Publicly available data such as nonrevenue water, pressure management, energy demand, network digitalization, and sludge utilization were used to explore the internal factors. The external environment was considered through the legislative and socioeconomic framework, climate change, etc. Finally, the credibility of the conclusions was verified in workshops with the water utilities. The key positive outcomes were that the external factors favor the shift to circular systems, while the major weakness, i.e., the aged infrastructure, is actually a good opportunity for the implementation of modern and circular solutions. The efficient collaboration of water utilities with other actors is a precondition for the development of a sustainable market for “circular” products.

This study aims at improving the existing algal-based wastewater treatment technologies by overcoming some of the major drawbacks of these systems such as large required land area, culture contamination and energy intensive algal harvesting. The experiments were carried out in an open photo-sequencing batch reactor (PSBR) at a laboratory scale for nearly two months. A specific strain ACUS00207 of the aero-terrestrial green microalga Klebsormidium nitens (Kutzing) Lokhorst was used. The strain is native to Bulgaria and belongs to a species that has never been used before in suspended growth systems for wastewater treatment for phosphorus removal. The culture of Klebsormidium nitens showed promising results: phosphorus removal rates (PRRs) ranging from 0.4 to 1 mg TP L-1 d(-1), efficient settling properties and resistance to culture contamination with native microalgae. On the basis of the observed phosphorus removal mechanism of 'biologically mediated chemical precipitation/phosphorus precipitation', an innovative working mode of the Sequencing Batch reactor (SBR) is suggested in regard to the potential for reduction of the Hydraulic Retention Time (HRT) and respectively the required land area.

Bulgaria is lagging behind in its investigation of per- and polyfluoroalkyl substances (PFAS) and consequently in its contribution to the global database and knowledge. This study constitutes the first assessment of PFAS in untreated leachate from regional landfill sites, which are a customary...