In the treatment of sludge, it is primarily energy use that contributes to climate change in the form of greenhouse gas emissions during energy production. In transport, fossil fuel combustion contributes to environmental impact in the form of climate change, eutrophication and acidification. When deposited, transport is also carried out in some cases, but the sludge itself also contributes to climate change, eutrophication and acidification.
The treatment process uses some method to dewater and dry the sludge. This method could be mechanical or thermal. These two types require different amounts of energy. The energy often comes from electricity and is produced in different ways depending on where in the world it is produced. Swedish electricity mix produces 13 grams of carbon dioxide equivalents per kilowatt hour while Nordic electricity mix produces up to 40 grams. This contributes to climate change (https://energiradgivningen.se/klimat/miljopaverkan-fran-el).
Transports are a big part in the handling of sludge. Both between treatment plant and disposals and with the transports of polymers. The transports are often carried out with fossil fuels such as diesel, when combusted fossil fuel reacts with air and form different types of compounds. These affect the environment in different ways. Greenhouse gases that are created such as carbon dioxide, methane and nitrous oxide affects the climate while oxides and ammonia contribute to eutrophication and acidification (https://energiforskmedia.blob.core.windows.net/media/17907/miljoefaktaboken-2011-vaermeforskrapport-1183.pdf). Methane and nitrous oxide have a bigger impact on climate change than carbon dioxide. Methane contributes 25 times more and nitrous oxide 298 times more (https://www.naturvardsverket.se/Stod-i-miljoarbetet/Vagledningar/Luft-och-klimat/Berakna-dina-klimatutslapp/). These greenhouse gases are important to consider.
The emissions that occur with disposal are connected to which type of disposal method is used. Transports are used here too but the sludge itself is a big contributor to the environmental impact. For example, when the sludge is used as fertilizer in agriculture, there are three main steps where there are environmental impacts. First, the sludge is stored to await the distribution onto the field. The emissions are mostly methane that are created in anaerobic conditions inside the sludge pile and is greatly affected by the water content and nitrous oxide (http://www.jordbruksverket.se/download/18.3421fb8e1634d8e3920b1d47/1526305320780/Emissionsfaktorer%20till%20utv%C3%A4rdering.pdf). The nitrous is formed closer to the surface of the sludge pile where there is access to oxygen. The emissions when distributing the sludge on the field, the second step, are mostly that of greenhouse gases and oxides as fossil fuel is combusted in the engine of the machine. Different types of fertilizer demand different amounts of fuel (http://www.bioenergiportalen.se/?p=5737). If the sludge can be spread with a spreader for dry fertilizers, the fuel consumption can be lowered to an eighth of that of a method for wet fertilizers such as manure. The third step is when the sludge lays on the field. Here the main emissions are the formation of nitrous oxide and the contribution to eutrophication. Both phosphorus and nitrogen contributes as phosphorus leaks out of the field into waterways and nitrate and ammonia forms from the nitrogen in the process of nitrification and denitrification (https://www.svensktvatten.se/contentassets/fdb93e09013543e6a84dff4e98289098/svu-rapport_2016-13.pdf
The polymer usually used in the dewatering process are polyelectrolytes. The most commonly used electrolyte used in Sweden is polyacrylamide, PAM, that uses a cationic monomer to flock the sludge. The flocking of the sludge particles helps the dewatering process in mechanical dewatering methods. PAM is bound in the sludge and still resides in the final product after the process. These polymers have not yet shown any signs of being harmful when used in agriculture. Though it has been shown that the polymer accumulates on the field as they decompose very slowly or not at all. This could be significant in the future (http://vav.griffel.net/filer/VA-Forsk_2003-40.pdf).
PAM also needs energy and chemicals to be produced. If the use of PAM was decreased or completely removed all the environmental impacts associated with PAM production would be removed.
The Freeze Dry Unit do not need any PAM to dewater the sludge as the process is not a mechanical one, such as a screw press or centrifuge. Instead, a natural process of freezing is used. This separates water and sludge particles. This results in a better final product without microplastics. The Swedish Environmental Protection Agency is now working on reducing the spread of microplastics in the environment (http://www.naturvardsverket.se/Miljoarbete-i-samhallet/Miljoarbete-i-Sverige/Uppdelat-efter-omrade/Plast/Mikroplast/).
Dewatered sludge have a dry solids content of 20 to 30 percent. This means that the biggest part of the weight and volume consists of water. Studies have shown that the sludge volume decreases up to 80 percent when it goes from a dryness of 20 to 95 percent. This results in fewer transports as more dry sludge fits on a transport.
As the Freeze Dry Unit is totally free from polymers, this also lowers or completely reduces the polymer transports.
Fewer transports also have an indirect effect on the environment as the wear on roads and trucks are reduced as well as noise from the trucks when they transport.
The cultivation in the world increased dramatically since the use of inorganic fertilizers started (https://books.google.se/books/about/Principles_of_Environmental_Chemistry.html?id=LFTxgCjZ0pgC&redir_esc=y). A growing population on the earth demands even more farming for food (http://www.jordbruksverket.se/amnesomraden/handelmarknad/allmantomhandelsochjordbrukspolitik/ulandsfragor/globalamatproduktionenbehoveroka.4.5da42c07159ce43e6207796d.html). To meet this high demand, effective ways to supply nutrients to farmlands is needed. Phosphorus is one of the most important macronutrients for farming. Deposits of phosphate rock has decreased which has led to the need to find new ways to effectively recycle phosphorus (https://books.google.se/books/about/Principles_of_Environmental_Chemistry.html?id=LFTxgCjZ0pgC&redir_esc=y ). In sewage waste there are lots of nutrients. A lot of them are bound in the sludge after wastewater treatment. Sewage sludge is a key source to be able to recycle phosphorus and utilize it in agriculture (https://www.naturvardsverket.se/Om-Naturvardsverket/Publikationer/ISBN/8800/978-91-620-8808-8/).
A method that can be used to recycle phosphorus and at the same time get rid of a lot of unwanted substances is to extract the phosphorus from the ashes after the sludge is incinerated. To do this effectively, the ashes must be relatively concentrated. This means that the extraction is disfavoured if the sludge is burnt along with other materials. To get an effective extraction, the sludge should be incinerated by itself (https://www.ivl.se/download/18.343dc99d14e8bb0f58b76ab/1454339645269/B2184.pdf). To do this, a dry solid content of at least 60 percent is needed. Below this, there is a need for a support fuel to incinerate the sludge (http://vav.griffel.net/filer/VA-Forsk_B_2003-102.pdf).
Sludge from wastewater treatment also have other qualities than a high phosphorus content. There is also a high nitrogen content. Nitrogen is also one of the more important macronutrients (https://books.google.se/books/about/Principles_of_Environmental_Chemistry.html?id=LFTxgCjZ0pgC&redir_esc=y). The nitrogen is not as bound in the sludge as the phosphorus and some of it ends up in the reject water from the dewatering and drying process. Despite this, a lot of nitrogen is still present in the sludge and can be used as a fertilizer (https://www.naturvardsverket.se/Om-Naturvardsverket/Publikationer/ISBN/8800/978-91-620-8808-8/). When incinerated, the nitrogen is lost, so to use this, sludge has to be directly applied on farmlands (https://www.ivl.se/download/18.343dc99d14e8bb0f58b76ab/1454339645269/B2184.pdf).
An important quality of sludge is that it contains humus-forming substances. These are needed to create a good soil quality so that the soils keep water and nutrients. With any bad quality of soil, nutrient leaks out to waterways and contributes to eutrophication. This increases the need to supply more fertilizers each time (https://books.google.se/books/about/Principles_of_Environmental_Chemistry.html?id=LFTxgCjZ0pgC&redir_esc=y).
Heavy metals are also present in the sludge. These are often bound to sludge particles and still reside in the sludge after dewatering and drying. Metals don’t decompose in nature and are still there after a very long time and can leak to surrounding land and water. Fish and other organisms are greatly affected by this as they become toxic and the metals wander up the food chain. In food that has been grown on farmlands with heavy metal content, these metals can later be found in the food (http://www.naturvardsverket.se/Sa-mar-miljon/Manniska/Miljogifter/Metaller/ ). Incineration of the sludge facilitates purification as heavy metals can be purified from the ash using various methods. The phosphorus can then be recycled as fertilizer without the spread of heavy metals (https://www.ivl.se/download/18.343dc99d14e8bb0f58b76ab/1454339645269/B2184.pdf).