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March 18, 2003 – Bonn, Germany. A lethal occupational accident occurred at the Bonn Salierweg sewage treatment plant when a technician fell into an overflow sewer and was immediately pulled into a 30 cm (11.8″) diameter drain. Rescue efforts were initiated immediately, but had to be terminated due to gas warning. However, when rescue teams continued the search, the body remained undiscoverable.

Fig.1 The Bonn Salierweg sewage treatment plant.

48 hours later, the cadaver was found in an adjacent digester tank after being transported between the overflow sewer and the digester tank through a 120 m (393ft) pipeline with several 90° bendings and branch connections. The cadaver showed marked signs of advanced decomposition caused by anaerobic microorganisms in the 37° C (98.6°F) biomass environment in addition to massive trauma.

Occupational accidents, often presenting with lethal outcomes, are a rarely reported issue in forensic literature. However, these incidents are part of medicolegal casework with special regard to reconstruction, liabilities and insurance law-related issues, respectively.

Fig.2 Overview of scene of accident. Digester tank tower with overflow sewer on top platform (green arrow). Adjacent secondary digester tank (scene of recovery, red arrow).

Fig.3 Digester tank tower with overflow sewer on top platform (green arrow). Adjacent secondary digester tank (scene of recovery, red arrow).

Case report

After refurbishment, the Bonn municipal sewage plant major digester tank tower (9200 m3 total volume) was started up again. Therefore, it was filled with 37° C water and biomass (sludge slurry; pH 1⁄4 7.2, organic acids 1.83 mval/l, 31% CO2, 69% CH4) to start up the decomposition processes.

In order to monitor the start up, technicians had performed several controls of the biomass. Due to safety prescriptions, the tower platform of the digester tank (Fig. 2-3) was only allowed to be entered by two people, equipped with gas (methane, CO2) warning devices.

Fig.4 Digester tank tower top platform with overflow sewer (green arrow).

At about 1 p.m., a 42-year-old female and a male technician entered the platform again to check the current sludge slurry composition and biomass temperature, respectively. When the male technician looked into the tank through an inspection window, his colleague presumably wanted to draw a sludge slurry sample for analysis from an overflow sewer (Fig. 4). For this reason, she had used a small bucket fastened on a rope, which she had wrapped around her right wrist. The male technician later reported, that he had heard a scream and as he turned around, he saw the legs of his colleague disappear into the overflow sewer.

The female technician had put down the bucket into the sewer to take a biomass sample and had subsequently been pulled into the sludge slurry, pouring out of a big pipe into the sewers drain (Figs. 5 and 6); i.e. the bucket had been swept along with the biomass pouring out of the overflow pipe, pulling the rope and technician, respectively, into the sewer. As the male technician had reported, that he had seen the woman’s legs disappearing into the sewer, the possibility could be ruled out, that the female had climbed down into the sewer to take a biomass sample and subsequently had lost consciousness due to gas and fell into the sludge slurry.

Fig.5 View into overflow sewer, notice steps on the wall and 30 cm diameter drainage pipe on the bottom. The overflow pipe on the right (yellow arrow). According to the witnessing colleague, the sludge slurry foam layer had reached the fourth step.

The male technician immediately climbed down a few steps in the sewer, but he could not find the body in the foamy and turbid wastewater, as the foam layer was approximately 0.5m (1.6ft) high. Moreover, his gas warning device alarm went off, therefore, he had to cancel the rescue attempt and left the sewer. Fire rescue and emergency medical teams were called to the scene immediately, but the body could not be recovered; after the wastewater had been drained from the overflow sewer, it was found empty (Figs. 5 and 6). Therefore, it was concluded, that the woman had been pulled into the 30 cm (11.8″) diameter outlet on the bottom of the sewer into the sewage plant pipeline system connecting the tanks and pools.

Fig.6 Overflow sewer, right side wall view. The overflow pipe was located on the wall above the drainage pipe. Most probably, the sample bucket fastened on a rope, attached on the female technicians right wrist had been suddenly swept away with the biomass pouring out of the pipe into the sewer’s drain, pulling the technician into the overflow sewer.

Due to the fact, that the major digester tank tower had been started up again, it was initially not clear, which valves of the pipeline system had been open at the time of the accident, additionally complicating a precise search for the missing body.

At that point, criminal investigators contacted the Department of Legal Medicine at University of Bonn to get answers to the question about the state of decomposition in the sludge slurry environment at temperatures about 37–39° C, to exclude the possibility that the body could become completely decomposed, making a successful recovery impossible. As there was little information available with regard to such kinds of accidents, based on general knowledge on decomposition in similar environments, a full decomposition within a 24 h time period was not expected; especially a complete decomposition of the skeleton (as feared by investigators and rescue teams, respectively) could be ruled out.

Fig.7 An example of a sludgy anaerobic digestion foam.

At noon the next day, employees noticed the body for a few seconds in the foam layer covering wastewater in an adjacent secondary digester tank (2500 m3 total volume), 15 m geodetic level below the digester tank, from where the body was rescued by fire rescue divers 48 h after the accident. It was concluded, that the cadaver had been transported through a 120 m (393ft) pipeline system, being the only connection between the two tanks, with a minimal diameter of 25 cm (9.8″) (discharge valve) and at least two 90° bendings and branch connections and a pressure difference (suction) of approximately 1.5 bar (1.5 x 10⁵ Pa). Subsequently, a medicolegal examination and autopsy of the cadaver was performed.


Autopsy findings

The cadaver was 172 cm (5.64ft) in size and weighed 54 kg (119lbs); the clothing had gone lost, except for the trousers and the shoes (Fig. 8). No livor mortis were observed, rigor mortis was completely resolved.

External examination revealed signs of advanced decomposition and putrefaction (Fig. 8), all the hair had gone as well as all finger nails and toe nails (Fig. 9). The skin was brown-greyish discoloured and showed large-area superficial loss of epidermis in small crater-shaped formations, most due to bacterial decomposition assumably (Fig. 10).

Fig.8 Overview of cadaver after recovery from secondary digester tank. Note torsion of the torso and extensive bloating of soft tissues.

Fig.9 Left hand. Note missing finger nails and discolouration due to bacterial decomposition; also missing skin above prominent joints, caused during transport through the narrow pipeline system.

The soft tissues were markedly bloated, as well as the abdomen; the fat tissue showed marked oily transformation. Large-area oblique superficial parallel streaky mark formations of the epidermis according to the wall of the pipeline system were noted on the whole torso (Fig. 11). Moreover, compound fractures of the left clavicle and humerus were found (Fig. 12). The right hand had been torn off. The thorax showed signs of massive trauma: the left thoracic cavity had been opened extensively, showing multiple serial rib fractures on both sides. The lower part of the cervical spine was missing, leaving the skull attached to the torso solely on a skin flap. A fracture at the base of the skull was also noted. Furthermore, multiple other extremity fractures were observed: lumbar cervical spine (torsion fracture), comminuted pelvic fracture, bilateral femur fractures. Both lungs, pancreas, spleen and the left kidney were missing.

Fig.10 Brown-greyish discolouration of the skin. Large-area superficial loss of epidermis with small crater-shaped formations, most probably due to anaerobic bacterial decomposition.

Fig.11 Large-area oblique superficial parallel streaky mark formations, caused by transport through the pipeline system on the torso. Note umbilicus; weak remnants of tattoo in the left groin (blue arrow).

As all the skin and soft tissues were brownish discoloured and markedly decomposed, the lesions could not be doubtlessly judged as vital or postmortal with regard to haemorrhagic borders. Microscopic investigation of internal organs revealed signs of advanced decomposition; in most cases, the organ structures had become unrecognisable. Toxicological assays of autopsy specimens for drugs of abuse and ethanol applying routine methods revealed negative results.

Massive polytrauma, most probably in combination with drowning in wastewater and asphyxiation by gases was ruled as cause of death. However, the extent of these single factors contributing to death could not be determined, as the lungs were missing.

Fig.12 Detailed view of upper part of the body and head. Total loss of hair, bloated lips, loss of teeth, compound fracture of left clavicle and humerus.


The reported case demonstrates two major facts: intriguingly, the warm anaerobic biomass containing wastewater environment—consisting of highly active anaerobic decomposition bacteria—caused marked and advanced decomposition and putrefaction within a relatively short period of time. However, these processes did not cause a full decomposition of the corpse, as initially assumed by the rescue teams. On the other hand, the cadaver exhibited signs of severe (poly-)trauma, i.e. multiple injuries and fractures, caused by blunt forces during the passage of a curved narrow pipeline system, connecting the location of accident and the location of discovery and recovery, respectively.

Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55 °C (131°F), or mesophilic, at a temperature of around 36 °C (96.8°F).

Fig.13 The Bonn Salierweg sewage treatment plant.

News report

42-year-old woman disappears in a sewage treatment plant – March 18, 2003

In an accident at the Salierweg sewage treatment plant on Tuesday, a 42-year-old employee from the city of Bonn is feared dead after falling into one of the overflow sewers.

The fire brigade and special divers searched in vain until dark. The search will continue on Wednesday.

The 42-year-old and another employee were doing a visual inspection of the digester after routine service. At around 1:20 pm the coworker heard screaming and saw his colleague’s legs disappear into the shaft.

A large-scale search effort was immediately started. Fire brigade, rescue workers, divers of the Red Cross and specially trained fire brigade divers from Cologne are at the scene. The police and the State Office for Occupational Safety are also on site.

The digestion tower will be pumped completely empty on Wednesday. “It takes a total of two days. And we don’t yet know what to do with the mud.” 10,000 cubic meters have to be moved.

Fig.14 Bonn Salierweg sewage treatment plant. Firefighters at the scene.

News report

Sucked 120 meters through the pipe system – March 21, 2003

The city of Bonn has lost a committed employee. After a two-day search, the 42-year-old sewage worker, who fell into a shaft in the plant on Tuesday afternoon, has been found dead. Employees laid flowers at the scene of the accident in the afternoon. They remembered their colleague at a memorial service.

On Wednesday evening at 10:20 p.m., an employee of the sewage treatment plant discovered the lifeless body in a post-digester and not in the actual digester. There the meter-thick foam blanket arched over the sewage sludge at one point. Underneath was the woman’s body.

The woman must have been sucked over 120 meters through the sewage treatment plant’s pipe system – starting from the site of the accident through a pipe that is only 30 centimeters thick and which first leads into a machine room. From there on through the pipe system and finally into the post-digester, which is only a few meters above the ground from the large digestion tower. At the narrowest point, the pipe is only 25 centimeters in diameter.

The two digestion towers are connected to one another by these pipes. According to the principle of communicating tubes, the different water levels would have led to a strong suction that reached a speed of three meters per second. According to a spokesman, the police’s investigations are ongoing.

Fig.15 Google Maps aerial image.

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