*3.4. Risk Assessment*

The modified risk graph presented Figure 3 visualises the assessment of the risk of portable lithium-based batteries. Two specific scenarios were chosen representing the observed areas of (1) collection and (2) treatment of waste from Table 8.

**Figure 3.** Modified risk graph for the two scenarios of Table 8: Waste collection (a small fire in a collection vehicle, CV) and waste treatment (a fully developed fire in a treatment plant, TP).

Keeping the grading of the probability of occurrence in mind, the various probabilities do not lead to different results in the risk graph, whether residual waste or lightweight packaging waste; however, they vary considerably in Table 8.

### **4. Discussion**

A broad spectrum of potential hazards of batteries was considered within this study's investigations and it is presented in Table 5, being exemplary for residual household waste. However, from a holistic viewpoint, in complex systems, such as waste management systems, it is unlikely to cover all of the possible hazards that are posed by lithiumbased batteries.

As [25] stated, risk matrixes may be subject to biases or errors. A risk matrix can compare only a small fraction of more or less randomly selected hazards due to their low resolution. In general, that fact has to be kept in mind when risk graphs are created or the basis of decision-making.

The scenarios that were observed in detail reveal that the risks are either in the ALARP region (yellow area) or in the region of inacceptable risks (red area of the risk graph). For the latter, measures of risk reduction are inevitable.

Fires in waste collection vehicles have become a well-known phenomenon in recent years. The smoke of the fires is usually recognised by the vehicle drivers in an early stage. An unspoken best-practice measure of the respective vehicle driver is to choose a known asphalt or concrete surface that is big enough to unload the burning waste (e.g., parking area of a nearby supermarket) and call the local fire brigade. The waste is usually pulled apart during extinguishing in order to speed up the subsequent fire-fighting operations and to locate the origin of the fire.

The different results for RHW and LPW indicated that waste shredders are the main hot-spot for battery-caused waste fires. In contrast to shredders, bag openers in LPW sorting plants are not crushing the waste. That is also confirmed by a recently published study [26], revealing high numbers of battery-caused ignitions after pre- and post-shredders in treatment facilities for RHW and commercial waste.

The comparison of the data of potential fire incidents per year with values from literature allows for an overall evaluation of these results. In contrast to the statistical data of waste fire incidents [1,27], where relatively low numbers are determined battery-caused fires, the results of this study seem to be very high. The data of [1,27] are characterised by high shares of unknown fire causes, which probably underestimates the relevance of battery-caused fires. In contrast to the high numbers of heat-related threshold exceedances in waste shredding machines found by [26], which are caused by batteries to a large extent, the results of this study seem appropriate. Autischer et al. [26] detected 0.1 to 1.5 threshold exceedances per eight-hour-work shift, depending on the respective measuring point in one of the two observed treatment plants. That means 260 to 1.580 threshold exceedances per year and treatment facility. In comparison to the results, it should be noted that (1) both treatment plants process significantly less than 100,000 tonnes per year and (2) temperature-related threshold exceedances are detected—not fire incidents.

### **5. Conclusions**

In this article, the main influence factors on lithium-based portable batteries' risk were investigated in their end-of-life phase. The degree of damage that was happening to portable batteries was investigated and compared for different waste streams. After that, the hazards and potential risks were modelled and assessed, including the present and previous studies' findings.

The results included a detailed analysis of the damage degree, the distribution pattern of influencing characteristics of portable batteries, and profound risk analysis and assessment.

The assessment reveals that the risk of lithium-based portable batteries is significantly too high, which makes it difficult to maintain modern waste management in a sustainable way. Primarily, municipal solid waste treatment plants are at increased risk. The probability that treatment plants burn to the ground is far too high, according to the available assessment. The increased number of major fires in waste management in recent years is clear and undeniable evidence.

No other substance or material has ever comparably endangered the whole waste industry. Hence, besides research and development activities for investigating and understanding the hazards and risks of lithium-based portable batteries, increased technological development and innovation efforts are indispensable for reducing the risk potential of end-of-life portable batteries.

In order to reduce risk, the waste sector has to aim to collect as many batteries as possible in the separate collection systems and take-back schemes, as only this collection system guarantees a damage-free return system. That requires increased effort in public relations and consumer awareness-raising. However, a 100% separate collection rate for portable batteries is highly unrealistic without a comprehensive deposit system. Hence, operators of treatment facilities have to find ways to (1) protect critical infrastructure and treatment processes (e.g., including new detection and extinguishing methods) or (2) preferably detecting and separating portable batteries in the course of their treatment processes.

Further research is necessary in order to gain more specific knowledge on the influence factors, where assumptions had to be made in the present study. Especially, the probability of waste ignition due to the thermal runaway of portable batteries has to be investigated.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2227-971 7/9/1/49/s1.

**Author Contributions:** Conceptualization, T.N. and R.P.; Data curation, T.N. and M.B.; Formal analysis, T.N. and M.B.; Funding acquisition, R.P.; Investigation, T.N. and M.B.; Methodology, T.N. and M.H.; Project administration, T.N.; Resources, T.N. and R.P.; Software, T.N.; Supervision, R.P.; Validation, T.N., M.H. and R.P.; Visualization, T.N.; Writing—original draft, T.N. and M.H.; Writing review & editing, T.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Austrian Research Promotion Agency, grant number 850753.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions.

**Acknowledgments:** The authors thank T. Bäck, L. Kranzinger, P. Puchbauer, W. Rübenbauer, S. Schlögl, B. Steiner, and the members of the Chairs working group 'Environmental Analytics' for their support in the sampling and sorting campaign. Furthermore, the authors thank the Austrian Research Promotion Agency for funding and the project partners for support.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

### **References**

