WaterSubject

The aim of this study was to evaluate the potential of different separation techniques for local recovery of faecal nutrients. Separation by Aquatron, filtration, flotation and sedimentation was tested in the laboratory. Only the separation of faecal matter from flushwater was investigated. For efficient nutrient capture, the system has to be combined with urine-diverting toilets.

In this study we found that the extraction of nutrients from the faeces to the liquid occurred rapidly. Therefore, to effectively separate the faecal nutrients and particles from the flushwater, the separation has to be performed locally, preferably at house level.

The Aquatron and the filtration gave a fraction of separated solids with 10% dry matter, which contained 70–80% of the incoming plant nutrients nitrogen, phosphorus and potassium. The other two methods investigated did not prove effective for local separation of faeces.

Using urine-diverting toilets, where all the urine is diverted, and collected and 70% of the faecal nutrients are separated locally, the potential for local nutrient recovery from the household wastewater is 88% for nitrogen, 75% for phosphorus and 55% for potassium, mainly in the form of directly plant available nutrients.

In household wastewater the main proportion of nitrogen and phosphorus originates in the urine fraction. Using a urine-diverting toilet, the urine is easily collected separately at source. The other fraction worth recycling, according to its nutrient content, is the faeces. The mass flow of the faecal fraction (faeces and toilet paper) is small, about 40–50 kg person^?1 year^?1, and to take advantage of this small mass for direct recycling the amount of diluting flushwater should be as small as possible. This can be combined with the advantage of the flushed toilet, if after a short pipe transport the faeces are separated into a dry fraction that contains the majority of the faecal nutrients and a liquid fraction containing the flushwater and some nutrients.

A system with the potential to separate, collect and recycle a large fraction of unpolluted human excreted nutrients can consist of a double flushed urine-diverting toilet, which flushes the urine and the faeces into separate sewers. The faecal particles and the toilet paper are then, after a short transport in the sewers, separated again. Some advantages of this system are that the convenience of the water-flushed toilet remains, that the nutrients can be recovered with just minor pollutants and that it provides an easy way to recover the faecal nutrients. However, one prerequisite for recovery of the faecal nutrients by using solid–liquid separation techniques is that these nutrients remain in the separated particles.

In this study the potential for solid–liquid separation of faeces and water was evaluated with respect to nutrient recovery potential. The study was done in two parts; the first part consisted of an investigation of what happens after the faeces are immersed into the water. The second part of the study evaluated the separation of faeces and water in a controlled environment in the lab, using different separation techniques. The aims of these studies were to investigate the potential of different separation methods under different conditions.

Compared to the other biodegradable household waste fractions, the faecal fraction (faeces and toilet paper) is small if the flushwater is excluded. The 30–45 kg, wet weight, of faeces that are produced per person and year correspond to 10–15 kg of dry matter. The amount of excreted faeces depends on the composition of the food consumed.

Of the faecal nitrogen, about 17% is contained in the bacterial fraction. About 10% is found as ammonia, from degradation of urea, peptides and amino acids. The remaining part is found in different organic compounds such as uric acid and different enzymes. The nitrogen in the faeces is to about 50% water soluble.

The faecal phosphorus is mainly found as calcium phosphates, but some phosphorus is found in organic compounds and some as soluble phosphorus ions. Potassium is mainly found in its water soluble ionic form.

The three main macroplant nutrients nitrogen, phosphorus and potassium are differently distributed between the urine and the faecal fractions. Of the food consumed, 10–20% of the nitrogen, 20–50% of the phosphorus and 10–20% of the potassium will be found in the faecal fraction, while the rest is found in the urine.

On average, more than 90% of the heavy metals consumed are excreted via the faeces and most of them have not been digested at all. When comparing the amounts of heavy metals in the urine and the faeces to the total flow from households, 10–30% of proven essential heavy metals (e.g. Cu, Cr and Zn) and less than 5% of Pb and Cd are found in the urine .

To utilise the potential of the high nutrient content and the low heavy metal pollution of the urine and the faeces, these two fractions have to be collected separately without being mixed with greywater. Using systems where these materials are source separated achieves this.

During the pipe transport of the faeces from the toilet to the separator, some physical disintegration of particles occurs due to vertical drops and turbulent horizontal transports.

The faeces pass through the pipes somewhat slower than water and therefore a certain amount of flushwater has to be used. If too little is used, the faeces will block the pipes. However, studied the transportation dynamics in the pipe using artificial standard faecal stool (NBS), therefore is it hard to predict what happens in the pipe with real faeces. The NBS is a plastic cylinder 38 mm in diameter and 76 mm long with a specific gravity of 1.05 g cm^?3.

Many methods are available for performing the solid–liquid separation. This study focuses upon four different methods: Aquatron separation, filtration, flotation and sedimentation. Of these methods, the Aquatron has been developed for this kind of separation, while the others are general separation techniques slightly adjusted in the lab for this purpose.

The Aquatron separates the solids from the liquid using a combination of a whirlpool effect, surface tension and gravity. The system is described in detail investigated on-site separation in a four-storey block of flats where, when undiverted urine was corrected for, more than 60% of the faecal nitrogen and almost 60% of the faecal phosphorus were estimated to be separated by two Aquatrons into a drier fraction of separated solids (SepS). In addition, 13% of the flushwater was separated into SepS. Due to relatively large flushwater usage, the dry matter content of SepS was low, 0.2%. With less usage of flushwater, a more concentrated fraction of SepS would probably have been obtained.

No studies on filtration of faecal water were found, but many studies have been performed on separation of farmyard manure. Filtration can either be performed by gravity or by using some kind of dewatering gadget, e.g. filter press, vibration or vacuum filtration.

In septic tanks a combination of flotation and sedimentation is used for separation of particles from sewage water. During the long retention time of sludge in these systems, in Sweden up to one year, major losses of nutrients occur due to mineralisation and extraction when the separated material is biologically digested.

When using sedimentation and flotation to separate manure, the SepS have a high water content. Another alternative for separating the faecal particles could be by using gas-induced flotation, where the particles are lifted to the surface by small air bubbles (0.01–0.1 mm) and then scraped off. One problem though is the decrease in density and surface tension caused by all the bubbles, which makes it more difficult to keep the larger particles floating, and if the bubbles are too big their own rise rate will prevent them from adhering to the particles.

The purpose of this study was, in a controlled environment in the lab, to determine how rapid the biological degradation, mineralisation and extraction was when water and faeces were kept together and to evaluate the potential of different separation techniques. The main focus for the study was to identify the potential for recovery of faecal plant nutrients to a dry fraction for a wastewater treatment system based upon local nutrient recovery.